The build is done, all that's left is to finish configuring, name it and maiden.
As promised, here's the full build-log.

And an overdue flight footage!

1. Parts

Parts choice is nothing revolutionary. The goal was to build an inexpensive freestyle quad to learn how to fly big racers.
I've never built or even soldered (almost) anything before this, so I also wanted to make it relatively simple - hence AIO FC.
Trusty Racerstars 30A V2, BR2205S motors and a HS1177 - all the basic stuff, except that the motors are supposedly a bit better than the original Racerstars while at nearly same pricerange.
A tiny VTX to top it off since my Rx is huge (I'll probably change it to X6B but for now FS-iA6B is what I had).

2. Build

a) Frame

I've started by assembling the frame. Make no mistake, it's not an easy task with the Martian II.
The arms have 0 tolerance between eachother, which might be a good thing because the frame assembled is firm and sturdy.
It comes at a great cost as assembling it took majority of the first evening and there's no easy way to do it.
Make sure to place the arms on the screws all at the same time - otherwise they won't fit.
Once you do that, hold them down to the bottom plate when placing the fiberglass plate on top. If you won't they'll raise just a tiny bit dis-aligning the screws and making it impossible to place the plate down.
I haven't used the included PDB, there was no need for it. I've replaced it with a spare fiberglass plate.

b) Flight controller

DYS F4 board is an amazing thing. Not only it has all the stuff you want nowadays - OSD, 5v BEC, current sensor - but it also comes with a pre-asembled XT60, softmounts and a buzzer. It's an all in one board, so you don't need anything else for your stack like a PDB.
I've started with XT60 and while I was at it I've added a capacitor - better safe than sorry.
Note that with capacitor soldered when making a continuity test the multimeter will give off a beep - not to worry, the capacitor is charging using the voltage from the multimeter. Keep holding it down until it's charged. If there are no shorts in the circuit the beeping will cease after a second or two.

c) Motors & ESC's

I've mounted the motors using longer screws (short ones are about 4mm, they were too short. provded are also 6mm ones) and laid out the ESC's. I figured it would be a lot easier to measure the wires if I mount the motors and solder them to ESCs first, then hold them down to the arm using some tape and measure the wires to the FC.
At this point I was left with a huge mess of wires and it's about time to clean it up - I've measured and cut the ESC wires leaving a fair amount of slack in case I mess up the soldering and will have to cut a portion to start over - so glad I did!
Before I soldered the ESCs to the FC I've slid some shrink wrap over them. I've also added the receiver wires in the meantime as I've decided to run them underneath the stack.
After a fair amount of work I had the power and ground ESC wires in place and a time came to sort out the signal wires.
These were a huge pain to work with. They have a really thick silicone wrap around them and there's not much of space left to solder them to the signal pads. This is most likely my own lack of soldering experience showing.
Once the signal wires were in place I tightened them down to the arms using some zipties and started working with the FPV gear

d) FPV gear

An easy part of the buld. Tinned the pads, soldered the camera wire and the VTX.
I've cut two cables off the VTX03 - it has a possibility to provide 5v to the camera, which I didn't need.
The wires on the VTX are quite short and in hindsight I should've extended them a bit. In the end it turned out that it was a bit difficult to place it where I want it.
I've also slid some heatshrink over the VTX - it's internals are completely exposed and I didn't want to risk shorting it to the frame.
The shrinkwrap I've used for ESC's was perfect - slid over so tight I didn't have to warm it and stretched nicely over the screen so that I still see the channel/band/power.

c) Frame's FPV bracket

Last step of the build was to assemble the FPV bracket and mount the HS1177 inside. Supposedly Martian II's camera mount is desiged for HS1177 and Swift.
This was a bit messy for a couple of reasons:
First of all, the threads on the sides of the camera were basically nonexistant. It felt like I'm threading those holes when screwing in the mounting screws which is no good with plastic bits.
Second thing is that the holes in FPV mount side plates are too small - this fortunately was not much of an issue. You can very easliy widen it accordingly by hand - whether with a drillbit or even a screwdriver. Be weary of the carbon dust - it's no good!
Lastly the back plate that should go on the back of your camera - a nice addition that you're supposed to mount at the back of your cam to adjust and "lock" the angle. In theory you remove the screws from the back of your camera, put the plate on the back and put the screws back in. Unfortunately RealAcc doesn't supply needed screws and the stock ones are too short.
I haven't been able to find the correct size so I used some thin doublesided tape.
On the other hand the side screws hold it quite firm on their own.

d) Finishing up

Oddly enough the hardest part was to lay out all the gear in the frame.
A big receiver combined with back-wired XT60, a capacitor and a VTX made quite an FPV sandwitch that I'm not too comfortable with but at this point there's not much I can do about it.
Ideally I wish DYS would use double sided power pads so that I could mount either the power leads or the capacitor on the underside.
In addition, as mentioned, VTX wires are a bit too short - but with all the struggle I went through I didn't feel like making them any longer.
I've used some double-sided tape to mount the Rx, VTX and some zipties and shrinkwrap to cover and mount the Rx antennas.

I've also added a HD camera mount - one designed for the Wizard fit in better than expected.
Back row of screws fit perfectly. The front was WAY off so I simply shaved it off and will use a ziptie or a doublesided tape to secure it.

At this point I've wiped beads of sweat from my brow and admired my work. Fly free, firstborn, go catch the wind!

3. Issues along the way

The first issue was my own stupidity and/or bad luck. I've fried two flight controllers before making it work.
At first I went with Betaflight F3 - it's 5v BEC died on me on it's own right after soldering XT60 alone. Turned out to be a bad batch.
Second was the DYS F4 from BG - right at the finish line a blob of solder dropped from my iron right on the side of the 5v BEC.
It shorted the regulator, some diodes among other things. At the very last moment I noticed it but was too late.
Bits of it got underneath some components and it was impossible to clean it off. I thought I did - I was wrong. Another one bites the dust.

Be SUPER careful when you solder an AIO board. Despite it's form factor it has a crapton of components squeezed together on a small PCB and even the tiniest amount of solder can short a bunch of things. Make sure to tin your wires away from the controller and be extra careful when tinning the pads. GND share a common, thick copper plate so it might take a moment to heat up and flow the solder nicely.
Take your time, mistakes are costly.
Make sure to not expose too much wire when stripping the silicon wrap. You ALWAYS need less than you think and exposed wire can easily short stuff on such a packed board.

Also get a decent set of allen keys. These darn chinese screws stripped literally all of mine - that's a total of 6. You need to tighten that frame hard.

4. Summary

Other than that I've had no further issues. The build was challenging for a first timer, but doable and mostly a great fun.
It's incredible what you can do wtih some wires and solder and the satisfaction is unbelieveable.
I've never even flown anything else than a QX90C and I really strongly believe it was a much better choice to build something instead of getting a RTF quad.
If you're also not sure - build it! Trust me. Price wise it's not so different, quality wise it's much better.
You'll learn how to fix potential issues and satisfaction will be so much greater.
With the amount of content on YouTube and help from the community you should have no issues whatsoever putting a build together.

I'm also open to name suggestion - something that relates to two fried FC's perhaps? ;)

At this point I'm waiting for the weather to get better to maiden it. I seem to be about done with the configuration but I'm a bit scared to do it indoors. I'll post some flight footage as soon as I can.

Thanks for all the help and cheers!