I get being disappointed that the series ended so suddenly, but I don't get being disappointed by the ending itself. Everyone's different, fair enough, but this was about as good of an ending as you could possibly get for a series ending abruptly due to the mangaka's health challenges.
We got to see them engaged, married, and with a kid. That's a lot to pack into two chapters. And two of those 3 things were more than just a couple frames.
What I would've been disappointed with is if it ended just with the two of them on the beach with Marin saying they'll be together forever (with some yandere vibes). That would've been really unfulfilling. But, that's not what happened; we got to see a look into their happily-ever-after future with them both being successful, having their friends and family still around them, and having a daughter. That's gold.
Would I have wanted more? Absolutely. Am I happy with what we got? Also absolutely.