Ok, just my thoughts on how this manga will progress (prolly wrong but I have a suspicion)
MC decided to seclude himself becoming a farmer/whatnot because he couldn't handle city life (and imo relationships with people)
He meets Knight-girl and starts changing certain lifestyles to help adjust to living with someone else.
For the ex part, she says "thinking" of getting married, meaning either someone's popped the question to her but is not yet sure about it (but in that regard, since when most people are popped the question its usually already asked as a yes or no so it might not be that), or it's leaning towards an "I know someone, but don't know what future she has with him." type scenario. And in that sense I have a suspicion she still has some feelings for the MC still.
My theory for this manga is that the story does not revolve solely around Knight-girl, but rather around on the MC learning to adjust to living with a "partner".
MC meets Knight-girl to help him with how he's lived his life, to help him grow out of his secluded lifestyle and start being more open with other people.
He couldn't stay with his ex because he couldn't understand what it was to live with someone at the time (as per flashback), but now because of living with Knight-girl, she helps him to become a person who thinks more of his partner's needs per-se.
Just my feeling for the finale, is that Knight-girl will have go back to her universe with no ill-feelings with MC, and MC grows up and moves on thanks to her help and becomes a more mature person. And at the end, he ends up back with his ex knowing more about becoming a better partner for her.
Again, I could be totally wrong, but I'm throwing this out there in case I'm right hehe.
TLDR; Manga is about farmer dude learning to live with a partner, and will eventually end up with his ex for the finale having learned to live with someone thanks to Knight-girl.