There aren't that many providers who have gmail-like labelling functionality, but luckily Gmail serves labels as folders over IMAP. May cause a lot of duplicates, though.
oranki
It's a really solid combo, but if you're not familiar with CoreOS I wouldn't change both at once. Meaning migrate the services to Podman first, then switch the OS. I've meant to switch from Alma 9 to CoreOS a long time, but haven't found the time.
I noticed you run Nextcloud AIO, just so you know, that's one of those "mount the docker socket" monstrosities. I'd look into switching to the community NC image and separate containers managed yourself. AIO is easy, but if someone gets shell to the NC container, it's basically giving root to your host.
Either way, you're going to have trouble running AIO with Podman.
I'm very much biased towards Podman, but from what I understand rootless Docker is a bit of an afterthought, while Podman has been developed from the ground up with rootless in mind. That should be reason enough.
The very few things Docker can do that Podman struggles a bit with are stuff that usually involves mounting the Docker socket in the container or other stupid things. Since you care about security, you wouldn't do that anyway. Not to mention there's also rootful Podman, when you need that level of access.
I'd recommend an RPM-based distro with Podman, the few times I've tried Podman on a deb distro, there's always been something wonky. It's been a while, though.
You need the G account to be able to install apps from Play Store, I don't believe the private space itself requires it.
Not sure if there's some Play "integrity check" on stock ROMs, but on GOS I was able to create the private space and download&install F-droid or other APKs just fine, without a Google account.
I haven't tried an OG Mastodon server, but currently running a GotoSocial instance, just for me.
With mostly the default retention etc. settings, the instance takes at most a couple gigs of storage space. If some image has been rotated, it will be refetched if you view the post again.
As for Federation, a single user instance is probably not a good idea if you're just starting with the Fediverse. Only content from accounts a user on your server follows will reach your server, including posts boosted by the people someone follows. I was already following about 150 accounts when I set it up, so I didn't really notice much difference in the home feed.
OG Mastodon can utilize relays, which will help with the lack of content.
For following topics, I made another user that follows some hashtag bots from fedi.buzz. The bots boost all posts with specific hashtags, so the posts reach my server.
If I were to do this again, I'd probably go with full Mastodon instead of GtS, just because I like the UI. There are other niceties too.
I think there's no way to keep the same domain while changing the underlying server software, without breaking federation. If someone knows a way I'd be really interested.