Self-Hosted Main

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.