I am in a similar place.

I picked up the BRRRR method book and feeling confident in setting it all up (we did a similar process for our Airbnbs).  

This post can be informational:https://www.biggerpockets.com/forums/51/topics/1088325-expla...as well as this discussion:https://www.biggerpockets.com/forums/51/topics/1225072-do-i-...A lot more of similar older threads can be found on this very forum.

Is anyone aware of any of a property management tool that allows PMs to give discounts to tenants who enroll in autopay (similar to how major US phone providers like do these days)?

Price appreciation followed a similar trajectory.

Similarly, if you put several Class D tenants in a Class A 4-plex, what do you think will happen to the property?

Honestly after I did my first deal I was in a similar position as you are.

I've been in a similar spot with family.  

I don't work in OK but in the state I am in, we have people do similar.

Of course, if we have another available unit that's similar to the one they were looking at, I'll pivot to that and offer to move their current application over to that unit as long as they would still qualify under whatever the new unit's rent is.