This is the opposite, fixing payment flow is something can be completely done without customer feedback. It’s q&a work.

You would want to test the response of customers on your product as well as your acquisition strategy, this is something you can’t fully predict, but still can get close if you spend a lot of time in that market

No one is going to bother with your side project if the first chance they attempt to use it, they simply can't. They'll say to themselves "oh well, I guess it's still a work in progress" and move on and forget about it. Posting a Show HN project is like an elevator pitch, you've got to capture the interest of your target audience quickly and be able to deliver at least a viable product. Not supporting PDF isn't a huge issue, that's just a future feature but not even allowing someone to use it is a failure even if the tech is sound.

This thing looks cool and if I was in real estate or apartment management, I'd probably be interested in trying it out. But if I can't, I'm just going to forget about it since I've probably already got a solution for 3D renders. At the very least, the emails are collected so OP can send out a blast saying it works now and hopefully some people give it another shot.

Sure, you need to consider the V in MVP. You also need the M though.