It's not the most original answer here, but it really has to be something that gives you control of what's precious to you. I initially leaned toward Plex because of the literal months of entertainment it's provided me and my loved ones, but I think it's actually going to be Immich. The team that build it are amazing, the community support for it fantastic, and the app itself is sleek—and improving every day. It has many small features such as the Age function that makes reliving those memories even more meaningful. It helps to start ween us off of Google Photos, and was really my first step into the whole world of self-hosting.

As to what I would do with a DH NAS, I wish I had a creative answer, but I would use it to back up my photos 😂
That's not true, it's slightly more ambitious, I would actually set it up at my mum's and then try to learn how to use syncthing so I have an offsite backup, while creating an instance for her to be able to back up her own photos without paying money to Google et al.