Are you alright with more extreme content like bestiality, scat, and urination? If yes, check out the Slice of Venture series by Ark Thompson/Blue Axolotl, as well as the games by Butakoma 300g. I think most of them have nipple insertion.
Vitamin Quest - A game developed by Butakoma 300g. It sort of serves as the inspiration for everything else I'll post here.
Vitamin Plus - A game that was technically developed by Butakoma I think, but mostly known for Blue Axolotl's re-imagining/re-release with similar characters.
Slice of Venture - Sequel to Vitamin Plus
Slice of Venture 2 - Sequel to Slice of Venture
Slice of Venture: Origins - A proper remake of Vitamin Plus. It's currently unfinished.
Also, if you want extreme content but not nipple insertion or stomach bulge, check out these other games by the creators I mentioned:
Drop Factory
Naomi's Past - The backstory of one of the characters from the Slice of Venture franchise. Expected to be completed by the end of the month.
The Hunter
The Hunter 2 - Currently being developed.