These also fits into your description:
Enka - Zetsurin Ou to Yokubou Ouji
Harem de Hitori

They were a nice read but I am afraid those aren't either. 😔 Definitely not omegaverse or historical. It was set in modern times and the couple was manlier too. I think the mother in law said something among the lines "We will accept him but only as a bride." I am sorry but I don't really remember much!

still looking!

Do you mean "mistress" in the sense that the seme was already or concurrently engaged/married to a female? Or was the couple monogamous?

Was the seme using the uke to avoid engagement/marriage to a female?

No, the seme wasn't married/etc to a female and the uke didn't want to avoid marriage. From what I remember, the parents would only accept the uke as a mistress and forced him to be 'lady like'.

Hi! I'm pretty sure I'm wrong about this one [I'll keep my eyes out] but Heart Strings sort of gave that feel...

Actually, WOW. This manga is exactly IT! Thank you so much~~~ 😀 😀 😀

Totsugimasen kara can't be the one because both the seme and the uke are supposed to be manly. This manga's uke is cute.