When I "window shop", so to speak, I generally look at https://alternativeto.net and https://awesome-selfhosted.net, but when it comes to migrations, the news about new projects tend to reach me before I set out to look for them myself. A lot of the time it's through fellow self-hosters on here and among friends.
Most services I've run so far have been long-lived as long as I was actually using them. Out of a couple dozen services there was *one* conscious replacement (Gitea->Forgejo, echoes of the perceived commercial takeover drama rang very far) over nearly 10 years I've been self-hosting things. Most changes are removals of the unused services and additions of new things. Also, swapping client apps, while servers stay put, thanks to open protocols.
So I've come to find upgrades to already deployed software to be enough.
But I know self-hosters for whom it's much more of a hobby than it is for me. My hobby/infra ratio is somewhere near 30/70.