You need agreements on how to evaluate hands

Playing online with a trustworthy partner, but one with whom I have few agreements other than that we are playing 2/1 udca, I pick up:

What do you open?

I opened 1NT and heard back 2NT.  In the absence of any agreements, this should be invitational (9-10 high-card points).  Would you accept?

On the plus side, I have four prime cards and a 5-card suit.  The five card suit has no texture though. But since partner likely doesn't have the missing ace, he's bidding on quacks and that might be the reason for his reluctance. I'd love quacks! He may even have a bad 11 points.  I think I have talked myself into raising.

I raise to 3NT.  Lefty leads the 6 of hearts (presumably 4th highest) and I see:

Looks like partner has stretched his holdings as well. He doesn't have quacks. Only jacks. What are my chances? I decide to duck a spade into west's hand and hope for a defensive error.  It doesn't work.  East gets in, leads a heart through my king and we are down 1.

Where was the problem? Firstly, opening 1S would have worked out better. Partner would have bid 2S and you need to now pass. However, 3NT is itself not such a bad bid. Give partner the Queen of spades or the Queen of diamonds instead of the jack of hearts and 3nt has a lot of play. The problem is in the lack of agreements and I'm not talking about bidding systems. You and partner also need to agree on how you evaluate your hands. If one side is counting length points and emphasizing Aces and Kings, the other side should discount unsupported jacks. Contrarily, if one side is counting jack-doubletons in NT games, the other side should also stick to Milton Work's high-card points.

The Too Late Now moment occurred even before we sat down to play.