I'm going insane trying to remember what the title is:
In the first chapter it was a guy who was originally from a Hi-Tech future fighting aliens? He then blows up everything with himself since all his comrades died and then found himself reincarnated into a fantasy world as like a 15/16 yo lord (blond haired). Then typical fantasy male lead stuff happens.
What I liked was that it was not setup for him to take over the country or something like that - it was getting sort of interesting where he meets like a Prince / heir to throne on the way while negotiating to get water from a lake from where I left off.
I do wish the seme was actually a villain and did it, but on the other hand it would have been really unsatisfying for the uke to forgive him that easily so idk. But the smut is sooo peak right now lol