That's normal. Just click New Game + and skip all the quests you've already done.
Thanks, but is there no way to just import one's last save?
No there is no oonable way to maintain compatibility with the engine they use, youll find that with most games in the genre tbh
I've done that with other games, so is this engine just badly designed?
part of it is just the nature of the games, there is a story tree that branches out and if it changes your old save will cause unpredicted behaviour