What should I do with spare PC?

Give it to me.

pretty basic

This spec blows my current gaming PC out of the water, let alone my homelab which is a 2015 Intel NUC running headless Linux and Docker.

This is either a shitpost or you have way too much disposable income.

You can check out awesome-selfhosted listed of softwares and see what you're interested in looking to self host. Common things to self host is a media player that host your movies/TV shows/ or music. A Google drive replacement like nextcloud. Another idea to look for is, whatever services you're using already , see if you can self host that yourself and see where that takes you.

Sell it. You don’t need it.

I'll take it off your hands. :)

Do a Mr. Beast kind of video where you just give it to someone for free but without filming the video.

Well.... Thats a beast.

Proxmox. Run multiple VMs. 2 VMs for ubuntu server, run all your containers through that, perhaps kubernetes? This way if your containers go down, they just switch over. Super nice.

Than with all the extra resources... I mean, home lab for fun. Spin up windows server. Diff Linux distro. Learn arch, etc.

So if your spare PC has those specs… what’s the specs of your main PC?

It can handle almost any service you might care to self-host - and with that much RAM, several at a time. You could run multiple VMs and still have breathing room.

But a much less powerful box can also handle most self-hosted services well. If your existing Pi is doing the job, I wouldn't switch. The 9900K will consume way more power, which is bad for the environment and your wallet.

Maybe make it into a testing station. Or donate it to a nonprofit. Or sell it. Or turn it into a living room gaming station, playing light games natively and streaming AAA games from another machine with Steam Link or Moonlight (in sleep mode when it's not in use?). Or give it to a family member. Or make it available to a neighbor via Freecycle/Buy Nothing/similar gifting networks.

Maybe a Jellyfin media server? GPU would help with hardware acceleration for transcoding.

That's a spare PC? I'm jelly.

With that much RAM, maybe buy a few NAS drives during Black Friday and use it as a NAS. Get an HBA card if you don't have enough SATA ports, and put TrueNAS on it. ZFS loves lots of memory.

It's overkill for a proxmox server that will likely sit idle

Use it as a streaming PC, buy a quick video capture card and use it to edit, stream and store footage while your main PC stays clean for larger games

This gets a bit subjective, because depending on your interests it could be a sort of "if it aint broke don't fix it" situation, so instead of telling you that you should, I'll just describe why the hardware would work well for it, and some benefits of it.

"and run everything off that 1 machine"

My stance is that a lot of people using dedicated NAS hardware would be surprised how much flexibility they find for the price after they get used to managing a hypervisor. There are plenty of reasons to use synology, but I think a lot of the synology people would feel less restricted once they've gotten used to spinning something up themselves and be more capable with scaling for the rest of their lives. A home hypervisor with a free core frequently already has a case with far more free space for drives than a hardware NAS and has free SATA slots. In almost all cases, someone isn't paying extra for any of that, and the only overhead that wouldn't be used is a CPU core and some ram. That compared to the price of a hardware NAS keeps me from wanting the hardware NAS to be a part of my life. So yes, I think there are some utilitarian benefits despite already having a solution to host those 2 services.

"What else can/should I consider hosting?"

This is already something you could do with the synology, but if you don't already have a plex server, having a share folder on my network that makes media accessible from every TV in the house whenever I click and drag a video or audio file into it has really added to the flexibility of my life. Recreation aside, one of the folders is "demos" and one of them is "training". Being in IT and having to watch a lot of demos from vendors about what their products can do, I've found my hoarding habits to be way more likely to refresh on the capabilities of a tool once that comes up in conversation again or there's some gap found in a capability. Most other uses I have are IT hobbies so it's harder for me to know what other selfhosted services you'd be into.

"Should I repurpose my old gaming PC to a Proxmox machine".

I'd just go ahead and try it and decide if you feel into it. Used old hardware is the best for that. 8 cores is plenty to dip your feet in, most use cases will only need 1 core per service on that CPU, and situation where you'd think you need more cores would also almost surely be a situation where you've proven to yourself that you're into it enough that buying hardware for the sake of a selfhosting habit is actually worth the money some day in the future. One benefit of having a hypervisor set up that I've noticed is that if you're lurking through this subreddit and see something come up, you'll be more likely to just give hosting a try on a whim. the added flexibility makes people more exploratory I'd say.

I do Debian with a kvm virtual machine, and tons of docker containers.

Llama2 for self hosted text-to-speech, speech-to-text, Immich’s AI-powered photo recognition. Hardware transcoding for Jellyfin. These are two things that need a good GPU and might be cool in a self hosted environment imho

Setup an XNS relayer.

https://xns.tech/relayer/

Me with my dying 1050ti and i5 6400 looking at this post: meme

PROXMOX: It would make a very nice Proxmox server that could host many VMs and LXC Containers. I have a similar configuration (though an i7) that hosts 2 Windows VMs, a Windows 7 VM, a Docker VM, and a second Docker VM running Kasm. Everything is accessible externally either public using Cloudflare Tunnels or restricted using Cloudflare Tunnels + Cloudflare Applications. I have a 300x300 Internet connection, and while it doesn't get a ton of use, it's always very peppy, even remotely.

Sell it and use the money for someting else