For speed and quality, I’ve had great luck with the fujitsu scansnap. Mine is much older. I’m not sure what types of improvements they’ve made (or have ruined). It was great for batch photos and documents. I have no experience with how it would do with slides or negatives, but for pictures, I think it was good.
I’ve had great experiences with HGST, WD, and Seagate. It’s less about the brand, and more about the “production run” and models. Backblaze always has an interesting blog yearly about HD failure stats:
https://www.backblaze.com/blog/backblaze-drive-stats-for-q2-2023/
I think Noble has some nice ones on the IEM side. I like their ear shaping, which matters a lot for a cable beyond quality/build for an IEM compared to a regular headphone.
I think you’ll probably be installing something like plex on a custom built or a synology.
It’s more of a question of how much time you have and budget. Do you want something with nextcloud like functionality out of the box? Synology isn’t a bad choice. It has all the apps for cloud backups, quotas, users, etc.
Do you want to toy with your system more? Roll your own and choose the best software for your needs.
I don’t know much German at all, but I see sinkende and think of “sunk cost” in English. Look up sunk cost fallacy.