After finishing the game, can I continue playing the post-game or do I have to start over to play again? I haven't finished the game, so I want to clear up this doubt.
You can rebirth to start a NG+ after finishing game so long as you gave birth to a valid baby and donated it to the church or brought it with you on the boat you keep your level and whatever is in your bank if you take that route plus you get a temporary extra trait point for taking the boat and an extra trait point (up to 5) for rebirthing
you can also rebirth by dying with the baby donated or in your inventory but you lose 20% of your total exp and don't get the temporary boat trait point
just note that abomination rebirths require you to sell the abomination babies to elise with the sell to elise option (at least 2 for human abomination 4 for abomination moot)then it becomes avaliable to rebirth into