I hear you, I went through the same journey. After getting Immich set up, I haven’t had to do a single thing to it. There are scripts that can turn your folders into Immich albums, so I just set up the docker containers, ran the script, and watched as everything was created for me without more labor on my part.

I hope somebody else has the solution you’re looking for, but if you don’t find it, I really think you should give it a whirl because it turned out to be way easier than I expected it to be.

Immich isn't complex, and I don't think it requires a separate database.

Edit: just checked and technically it does, but if you use the docker compose then it deploys it automatically and you don't have to think about it at all.

Edit2: if you really want something dead simple, yeah you could trivially write a tiny script to list and thumbnail images. I'm sure there are apache plugins for exactly this.

It does nothing but serve photo. Background tasks are minimal and many of them only run once on new photos.

You're likely not going to find something so simple and lightweight. Immich has some setup, but if you want a good gallery that's the way to go. Don't be afraid of the learning.

I feel like this is a case of not wanting to do the "difficult" thing because it sounds difficult when really avoiding the "difficult" thing is more difficult than just doing the "difficult" thing.

I was replacing the power steering rack on my P2 Volvo XC70 recently. I really didn't want to drop the subframe because that sounded like a lot of work. Eventually I realized dropping the subframe was the easier way to get the power steering rack out and, in the end, it was way easier to just drop the subframe. It was like 10 more bolts and wasn't that hard to put back. In this analogy, Immich is dropping the subframe, just do it.