Thank you for playing!
Here is a walkthrough, in case you're missing any:
https://steamcommunity.com/sharedfiles/filedetails/?id=2650602180
If you've unlocked all 13 and seen all CGs, that's pretty much the full content of the game. It's possible to get the [spoiler] different starting scene after you unlocked enough endings, but 13/13 isn't a requirement.