Did you also check out GoToSocial? It’s a very light Mastodon-compatible server, but comes without a user-facing GUI. So you need to use a client app.
However, I don’t know whether it can be easily migrated to from Mastodon.
GoToSocial is awesome. Some features are still missing, but the server is in active development.
As front-end I use Elk. It's selfhostable as well as publicly usable at https://elk.zone. It's labeled as alpha software but runs absolutely well.
That does seem like a good option for my use case, interestingly they note that no matter what I switch to I should change the subdomain:
It's not supported across the Fediverse to switch between implementations on the same domain. This means that if you run GoToSocial on example.org, you'll run into federation issues if you try to switch to a different implementation like Pleroma/Akkoma, Misskey/Calckey etc.
That doesn't look promising for moving the account without migrating because then I couldn't maintain my username.
Have you checked GotoSocial's docs on split-domain deployments?
https://docs.gotosocial.org/en/latest/advanced/host-account-domain/
It looks like something you would want. (I will admit I have no experience with it though)
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