I don't care about the age gap at all because I'm pretty sure nothing inappropriate will happen before she's of age and if everyone is of age then 10 or 20 years doesn't matter, in my opinion especially because the historical setting always suggests that people become adults and mature mentally much faster than nowadays.
Anyway, I'm concerned about how uninteresting the story is. Don't get me wrong, I dig the isekai genre from pure action shounen to cute shojo with an oblivious supposedly villainess so the type isn't the problem. It's just that the MC and her fiancé are so bland the same goes for everyone else. The premise isn't the problem but this is so boring. I'm currently reading manga like "the villainess wants to marry a commoner" where the title literally says it all and everything is simple but it's an attractive story despite its simplicity and even though nothing major ever happens, it makes me run to read the update.
This story is meh at best. I'm still going to continue following it because I hope the mangaka proves me wrong and things get better eventually and if not I can just unfollow it whenever