Hey quite a few questions which would each require a detailed answer, I will just try to share my thoughts and experience…

First of all, raspberry pi: Which Model exactly? 3B+? 4? How much RAM? In my experience, self hosting on a raspberry pi for a small number of users (1-3) works for most applications. Make sure to use a good power supply and regularly backup your MicroSD card and it will run for years without any problems. You might need to use a USB Hub with external Power supply if the HDD uses to much power for the pi USB ports. Also enable spin down while idle. Research a little about disk format, for Linux ext4 is a good start, but formatting a disk generally deletes all files and you must consider if the file system needs to be compatible with other then Linux like windows or Mac…

Regards the choice of software for photo backup:

In my experience nextcloud is good for file syncing, but not that good at browsing large photo collections. Also the mobile apps are not that nice to use, but that’s my opinion.

I would recommend to take a look at Immich which runs also very well on a raspberry pi, has great performance ux ui and mobile clients.

Regards Operating system: Yes, a clean fresh install with newest raspbian (=raspberry pi os) is recommended. You should enable ssh for remote management so you don’t need a monitor and keyboard connected to the pi.

I would also recommend using Docker and Docker-compose to install nextcloud and or Immich, its easy to maintain, update, migrate and restore.

Also you should get another HDD for backups, data corruption, bit rot and human errors are happening so you should backup manually every few weeks.

These are just some thoughts and by far not complete or the only ways but it may help you start. Also watch some videos or follow tutorials to get familiar.

Kind regards and have a good journey into the self hosting hobby, its great, you won’t regret it as long as you are willing to learn new things.

Its always DNS

Finamp, very polished, actively developed and cross Platform. Requires a jellyfin instance. But thats a nice diy project in it self, can only recommend

Are the external USB drives using a external power cable or just USB power? To me it sounds like you are hitting the Pis USB Power limit. You may want to get a cheap external USB dock with dedicated power supply