The issue isn't wanting to make the stimulus checks bigger, that's fine. It's the timing. I don't think there's any reasonable way to get that done before unemployment aid expires in a few days, and the eviction moratorium a few days after that.
If Trump was going to veto because the checks are too small, why not say it 2 weeks ago? All he said was that he wanted bigger checks, but nothing about vetoing a bill because of it.
EDIT: It is a veto-proof bill, so he may not have a lot of say, and he never said the word veto, so I'm just an idiot reading the news too fast.