You probably can! At least if your FCs are at least F4/F7 - F3 series microcontrollers are no longer supported. But the way Betaflight ‘upgrades’ work you’ll need to reconfigure the FC after the upgrade. And using the CLI dump made with an older firmware version isn’t supported - some options go away or get renamed, PID tuning options in particular should not be blindly copied between versions.

O3 is the best quality digital video system (made by DJI). The air unit (camera+digital vtx) is $230, and the goggles are $500. It can also record up to 4K at 120 FPS (in-goggle max quality is 1080p at 100FPS). Analog is definitely way cheaper (at least the vtx side), but I think most people who could afford digital have switched over the last couple of years.

DarwinFPV BabyApe is $80 and flies better than anything you’ve built 4 years ago due to improvements in Betaflight. Sure, if you want O3 you need to pay more.

Open the door sensor, locate the reed switch, remove it and connect wires to its PCB pads. Although it should be pretty straightforward to measure the minimum distance required between the magnet and the sensor and install the magnet just a bit closer, so that any cracking of the door opens the switch. If you use a weaker magnet, the sensor will be more sensitive to the distance, so you will be able to detect smaller gaps.
Side note: if the door doesn’t close when there’s a small gap, the fridge might not be level or your door auto-closing mechanism might be worn out.

It’s hard to recommend a controller, ergonomics are mostly personal preference. I would assume the target is thumbers, but in Stingerswarm’s review he had issues with button placement when using the thumb grip, so who knows… If you can buy both Pocket and Boxer from a place that allows returns, that would allow you to compare them and return the one that doesn’t fit your hands.

On goggles - depends on where you fly, Cobra X has a proper mixing receiver that should work better in high multipath environments, but for flying around a field EV800Ds should be plenty adequate. Cobra X has HDMI input, but I doubt you’ll end up using box goggles with external module if you decide to go digital, that seems pretty cumbersome…

Dronetag BS (€90, ~2g)

https://dronetag.cz/products/bs/

You’ll need to buy the antennas separately, e.g.

https://shop.dronetag.cz/products/44-combined-bluetooth-gnss-positioning-antenna-ufl.html

Here’s Bardwell’s take: https://yewtu.be/watch?v=eC6exFgu8IQ

Personally I’d hold off on buying the module(s) unless you have 107 gigs lined up…

Did the same with Trianglelab Dragon. Damaged the heatbreak as well when disassembling the hotend. Now I own a 1.8Nm torque wrench…

Only if flying under part 107