Save slots are just a hard cap from RPGMaker unfortunately. Keep in mind though that your saves are just files in the save folder: in theory there's nothing keeping you from copying them to a second folder, clearing the main one out and starting a new set. Bit of a hassle, but may help for some.
As for a gallery: a lot of people have expressed interest in it. What I did with the previous game to help a little was to publish a scene guide, so you could have some sense of what you had missed and how to get there if you weren't sure. I just haven't been able to settle on a way to do a gallery I'm happy with. Putting an option on the main menu requires an edit of it, and is universal rather than being save-specific. I could give an item that transports you to a room that lets you select scenes via interactible objects, this is how most RPG Maker games seem to do it. But I've never really liked the feel of that. I'm someone who cares so much about detail that I made sure the special stats in your status menu get crossed out during one of the bad ends for consistency. I don't like having an arbitrary meta item hanging out in the inventory.
At this point enough people have asked for it that I'm gradually coming to terms with the fact It'll have to happen eventually. Probably not until much later though, when all the intended scenes are finally in the game. Maybe I'll have some kind of intricate "secret" interaction code to unlock the item (that I'll share here of course), so you only get it if you deliberately seek it out.