As soon as you get below 100% (or whatever the required amount is) they're selling.

Initially I managed renting it out but as soon as I used a decent realtor it made it so much quicker and raised the rental amount.

What amount of cash flow per month would it be worth it to keep it?

But if you’re fixing it up you might actually increase the taxes even higher. 

Any "ward" in Houston should get some questions going, but majority of Houston based agents will push this ad nauseum especially the one's on here and have been the better part of the last 2 years. 3rd ward for MTRs, STRs, and travel nurses and 5th ward for fix & flips; but only an idiot would buy a 5th ward property without doing their diligence.

That is a tremendous amount of money for 2 agents.

Monthly payment to HELOC interest only will be $312 added to mortgage whatever the amount would be (e.g. $1200 for 150k loan).

We'll then write a promissory note for the amount, maybe attached by mortgage (if not, a lawyer said attaching a judgement lien upon default isn't too hard, provided no chapter 7 bankruptcy filing).Has anyone done this post closing?

I have done some Google research and articles point out concern for not being able to see the decking and determine if there are issues and fix them. 

Sometimes it might make sense to have full service, but if you are just fixing a drain once every other year then it probably won't.