Consider more than just rental income, such as, prior tenant history, civil and criminal background, employment history, number of occupants for the size of the home, desired move-in/move-out date, etc.

Technically true and if you are looking short term then you can not bank on it.

Background....I invested in two DST's in Jan of 2022. 

technically it happens all the time, but real estate is less liquid thus its more obvious. ever try and buy a stock at a specific price and that price is no longer valid because the price changed.

. $40 application fee - runs credit and background checks on tenants.

It is "technically" simple to transfer title to a property, all you need is a properly drafted deed, signed, notarized, and recorded.  

With an agreement: the Buyer is your client and the seller is the client of the listing agent , hence they do not technically owe you a commission.   

When taking applications, should I charge a fee to cover background, credit, and employment checks? 

It's not technically investing but it can help you increase your savings.

And tenants that pass muster - good credit scores, stable jobs, good income, no criminal background, no evictions, etc - generally are responsible pet owners.