I already used desec.io for my domains back when I had static IP blocks at home so I just used the dyndns api with ddclient to update them automatically for my dynamic IP.
A molex to PCIe adapter with an 850w power supply. I only have like 10 watts of headroom but usually only one gpu is running at a time so it works fine. I would have bought a better one but my dad accidentally bought 2 for another project so he had an extra one and it works fine.
I have been running wayland with sway for around 6-10 months (I forget when I switched). I have a 4-monitor setup and hated the default workspace management but swaysome convinced me to switch. I heard a lot of stuff about how manual tiling is bad but I actually don't mind it and kind of prefer it to automatic tiling from AwesomeWM which I used before sway, .
I have a 6700xt for Linux and an old 2070 that I pass through to a windows guest. I might get a usb card too to pass through but not sure if I have any more space in my mid tower for another pci card.
I would not suggest mastodon for such low powered hardware, its also overkill for a personal instance. Akkoma or GotoSocial would work much better on a Pi. The annual cost is pretty much just 3-15$/year for the domain name.
My encryption key dies with me and nobody will be bothered to save my drives and crack them.