Focus on services before devices. iCloud to Nextcloud, iPhoto to Immich, HomeKit to Home Assistant, etc. You can start using them concurrently, alongside Apple services so you still have that safety net while you figure things out. Most self-hosted services will have iOS/MacOS/TVOS clients so you can start using them now. Then when you’re ready to swap a device, you’re mostly ready to go.
I’ve been working on this myself after being all in on the Apple ecosystem since 2006. So far, I’ve moved to Jellyfin for my media, and made the Nexcloud and Immich switches. The most important thing here is that as you start to self host more important things like photos and documents, have a good way to back up that data.