My husband and I would divide and conquer. 

The way I advise my borrower's is to take the savings on your monthly mortgage payment and divide it by the cost to buy down the rate.

If you have a move-out every year, that would amount to a vacancy percentage of 14.3%; two divided by 14 (12 months tenancy, plus the two vacant months).

And if so, where would you draw the line?

When the partner’s draw payments ceased this CPA became involved selling Sponsorships” in a very dubious MLM scheme; he was trading in all the goodwill and reputation he had garnered in 25 years of practice to make a quick hit because of his financial position. 3.

One unit has tenants I would like to keep (it is much larger and draws much better rents).

The best thing you can do is really be on top of them with the draws.  

So a typical (around here) 1 year lease's fees would generate {.5X + 11(.1x) = 1.6X}  Divide that by 12 and you get an average of 13.33% of X per month as the LTR property management fee.An typical STR fee would be 25%, but there should be more vacancy.

Main Account - Checking account to set up transfers or draw money from for mortgage, insurance, taxes, etc. and I get a Baselane debit card (physical and virtual)2.

No, the “great divide” isn’t between residential and commercial, nor between owner occupied and rental.