I'm only building rental cabins which is a higher price point due to their ability to produce income. 

Aside from the responses above regarding the bank likely requiring the 20% funds to be yours, good luck finding any private lender willing to take a second position note on a 100% LTV for any %, let alone a tiny 3%.Now, banks do, at times, allow seller carried back seconds, but unlikely with no skin in the game from the borrower.

Vacation rentals in these mass-produced STR "neighborhoods" are really just a commodity, similar to a hotel room.

As far as a reasonable fee, I'd say it has to do with the value brought, and product level produced.

If you set them up as a "property manager" who was required to produce X amount of income to you (i.e. your rent amount) and then they'd get the rest. 

Once you pick one give it a serious dedicated effort, but if it's producing no fruit after several months you can always pivot and shift. 

In 10 years it is virtual certainty that a leveraged purchase today would have produced a good return.

That doesn't mean it isn't a good strategy - if the property still produces healthy cash flow after expenses (which includes future capital needs if you've deferred such), at a minimum you could be rolling the cash flow either back into the home to own it outright or using those funds to prepare for your next purchase. 

A marginal producer isn't likely to improve if you increase it's cost of finance.This is where you would sell and 1031. 

Or just go to whatever brokerage affords you the most flexibility and just start networking with investors and investor agents to build that support system.As a part-time agent the last thing you want is high desk fees or other junk fees, since you may not be producing a ton in the beggining.