Home Assistant. If you ever want to do home automation properly, this is the way. Works with pretty much anything—Zigbee, zWave, BT LE, MQTT—while keeping things manufacturer agnostic, local, private and highly responsive (your commands don't need to go through some server 3000 km away and won't have ugly 1 second latency as a result).
DAVx⁵ and Radicale to sync contacts and calendars between devices without snooping middle-men.
Syncthing to sync any files between devices. Works remotely, too, thanks to Syncthing relays.
Navidrome for your personal music streaming service.
Debian, Docker, Docker Compose and Portainer as the backbone to run all your services.