I    have a good amount of cash and have access to quite a bit more, yet that isn't enough, even with proof of funds.

Or, if you're happy to stay living there, but want as much immediately accessible cash in your pocket as a Lender would approve, you'd need to make sure your newly acquired investor mindset was in full bargain hunting / return-on-investment mode, because, your monthly repayments will increase significantly once you withdraw the maximum against a HELOC!

Also, the MLO method allows you to work in the bigger deal space, it gives you the opportunity to control and profit off an amount of units you could never purchase with 20,000.

There are some exemptions for not having to add an extra parking space for the ADU.

Equity, bought by the tenant in a property you control, is free money...to be accessed later.Concept #2:  When Einstein was asked, "What was the greatest invention of the 20th century"... he answered, "Compound Interest". 

-Rent estimates for retail space, especially vacant ones.

My rentals are all in a small town and the bank is close for everyone, I understand that this may not be a suitable solution for landlords with multiple properties in different locations.Did you just choose a bank that was most accessible for your tenants? 

I also have a portion I'm wondering if I could dig out and convert to living space, along with adding a walkout egress door.

Currently the rear of the garden unit is divided up by walls to create a furnace room somewhat in the middle of it, and a second closet-type room to access the furnace room.

If the space is in a great area, that might be all you need.