Some starting points
- photos: NextCloud
- git: Gitea
- BitWarden: Vaultwarden (even if you deploy this locally you want a SSL certificate as clients will refuse to connect otherwise)
I'd suggest using official docker images to get started as there’s plenty documentation available for all projects and experimenting is a bit easier when you can simply dispose a container without having to worry what’ll happen to your host OS.
As long as you run services locally on your Synology (assuming it supports docker) and don’t expose them to the Internet I’d encourage you to „just give it a try“.
Just don’t immediately start to rely on the services and run a dual strategy (NextCloud and iCloud photos for example) till you updated your container once or twice and feel comfortable troubleshooting issues with your stack. Nothing is more discouraging than having a service you need „right now“ being down and no idea how to get it back up.
It’ll be a long, fun journey. Good luck!