I just received a digital copy after we received the deposit and first month’s rent. 

We’ll only lend after we’ve met and gotten to know you just a little, you must be a full-time house flipper, and you must have a good deal; for which we provide specific criteria.Obviously, this only works locally, and it would be impractical if we had to deploy many tens of millions out of a fund.

Just ensure that your increases comply with local rental laws and regulations. 

Yes this strategy works great especially if the numbers don't work locally.

To get a good rental income, be sure to check who you rent to very well, write a clear rental contract, take a security deposit, make a smooth way to get rent, keep the house in good shape, think about getting landlord insurance, keep talking to your renters, and follow local rules strictly.

I just let Airbnb do their thing and do my normal due diligence on Airbnb guests like asking why they're coming, making sure they have a valid ID and try to avoid locals unless they have a good reason like home shopping

Your answer would be it depends on a million things.Like the specific property, its condition, location to desirable areas, local STR regulations etc.We would need much more detail to be able to help.

Do a thorough market research and consult with local real estate professionals.Good luck!

Inside a retirement account, you still get all the appreciation and rental income that adds up massively over years and the returns are usually well over double digits.  

My hope is to begin building my portfolio (probably fix/flips) this year while I continue to work with local investors on helping them build theirs.