Va-11 Hall-A is something that I would recommend to anyone looking for a not so serious visual novel. The characters, world building, and atmosphere are all excellent. It may or may not give you something to think about. Might not be everyone's cup of tea though
Kara no Shoujo 1 and Kara no Shoujo 2 are two very serious visual novels. If you were looking for something heavy to read, you would probably like them.
Umineko no Naku Koro ni and Umineko no Naku Koro ni Chiru are considered to be among the best visual novels ever created(Based on VNDB rating). Playing through both games would require a significant time investment though. If you ever find the time, I'd 100% recommend that you play them
I'd also like to take this opportunity to praise your VNDB list