I'm using an old thermaltake case right now for one of my PCs. Can hold at least 14 hdds, more with some better hotswap bays, but its a tallboi case

Why wouldn't it? A lot of people install games to the default location and the default location for a lot of things, games included, is C:\Program Files

It was something I was worried about when I had services exposed to the internet, but now that I'm behind CGNat, I'm not so worried because I have nothing exposed lol.

Plenty of enterprise equipment has 48V DC power supplies.

My work setup is Mains -> Rectifier shelf -> DC PSUs/Devices
Batteries are hooked into rectifier //

If you're ok with blowing away Proxmox and starting fresh then yes, that's something you can do.

I will be doing the same on a Dell miniPC soon but with smaller SSDs.

I prefer Ciscos, but TP-Links are also "fine".

I have a couple of FS.com switches as well and they work well.

If you just want video then HDMI capture card like the other replies.

If you wanted HDMI output and also input keyboard/mouse, another PC with a USB Crashcart device or PiKVM

I still have a 2RU Intel Rackmount hub with a fibre module.

Does it have any use? nope. Does it look cool? Absolutely.

Out of curiosity, why is VPN not an option?

I would be putting the FTP server onto its own subnet and VLAN and making sure it cant talk to the rest of your home network

I'm using an AMD Turion and running ~6 VMs on that. I used to run pfSense on it as well, but I had to take out the second network card to put in a HBA.

I shelved my big rackmount server/NAS in my 550mm rack

So. There are NVMe to U.2 Adapters, like this - https://www.mwave.com.au/product/silverstone-mua01-m2-nvme-ssd-to-u2-ssd-adapter-ac69201

That converts a NVMe SSD to something resembling a 2.5" SAS SSD.

Then there are SAS/NVMe Hotswap bays, like this - https://www.intel.com/content/www/us/en/products/sku/99083/2u-hotswap-8x2-5inch-sasnvme-combo-drive-bay-kit-a2u8x25s3phs/specifications.html

Then there are these cables that connect Occulink (PCIe) to SFF-8643 - like this one - https://www.microsatacables.com/oculink-sff-8611-straight-male-to-sff-8643-cable

Then there are PCIe to 4x SFF expansion cards like this - https://www.amazon.com/Expansion-Adapter-SFF-8643-Indicator-Breakout/dp/B0B7BHM954 - This will require hardware support for PCIe Bifurcation, but will provide you 4x SFF-8643 sockets per each 16x slot.

Do with this information what you will.