Just wondering if anything is missable such as side quests or cg scenes?

Well before the finale you can return and do those things you missed but there is a guide on steam that explains it better:
https://steamcommunity.com/sharedfiles/filedetails/?id=1541285040
This one is only about Evenicle 1 though i haven't reached the finale in 1 nor did i play evenicle 2.
edit: It is kinda spoilerish since it tells you which wifes you get.
Edit2: after finishing the game you can load a save file if you missed somthing and you will be placed before the last battle. I finished the game