It's a bummer that if the creature is killed in response, you don't the land or card draw either

This template doesn't actually fix the problem. This lets the ability trigger if there's no creatures in play, which is something I hadn't even thought about the first time, but the whole ability is still countered if the target creature is made illegal

Would a simple "you may" inclusion fix that? If not, what's the fix?

This starts to change the design a bit, but you could make it also target the player. As long as one target remains legal, the ability resolves to the best of its ability. The other option is having it "choose" a creature to gain hexproof rather than target it, but that's atypical.

>At the beginning of your upkeep, target player draws a card and may put a land card from their hand onto the battlefield tapped. Up to one target creature they control gains hexproof until end of turn.

(I swapped the order of the draw and ramp so that you have a chance to draw the land you want to play.)

I also think an on-board trigger that grants hexproof feels disappointing. Your opponent would need instant-speed removal to deal with the creature on your turn anyway, so they would probably just respond to the trigger. I think it would be better to be an end step trigger that grants hexproof until your next turn, to at least protect against sorcery-speed effects.