I am currently using DokuWiki via Docker Container. It's simple and works flawlessly

I can recommend Hetzner without any problems

As far as I know, there is no current arm Docker image for your Raspi 4. You could rent a cloud server. E.g. Hetzner. I use the CX21. It runs very efficiently for me.

1. You can subscribe to any community or add the instance to the Allowed Instances
2. Yes you can search the communities through your search e.g. !fediverse@lemmy.world an subscribe to it
3. My Instance is running on Hetzner I am using the CX21 (2 Cores, 4GB RAM, 40GB Disk Space) and its running really good (Its just me on this instance for now)
4. definitely yes 😁

Maybe have a look at PhotoPrism. You can install the app via PWA on their IPhone and upload their pictures through the webinterface. And it indexes images pretty well.

This tool shows pretty well how the system works Toolness