Unraid does support unequally sized drives, your parity drive just needs to be as large as your largest drive. It’s not true mirroring of course.
The easiest setup would be to create two RAID1 sets, this also gives your the highest chance of recovering data in case one or more drives die.
You might want to consider a typical desktop/tower with a consumer CPU. The noise and heat from a simple desktop PC, even of you have the case filled with 10 drives, is so much less than a server chassis.
If you can still find them, Intel Optane drives are great for caching.
I’d up the RAM to 64GB and run the extra stuff in Hyper-V that way it’s easy to keep your gaming OS clean and prioritize services.
Google ‘dual PSU adapter’.
There are two options:
1 - a 24 pin adapter that bridges the power-on pins to the other PSU
2 - a 24 pin adapter that takes a molex from the main PSU and uses a relay to power on the secondary PSU
Either works just fine.