@Robert Brown I also take the slow and steady save and pay approach with conventional bank lending and what helped me was refinancing into the name of the business so I was able to remove the debt on my personal credit report which made me more lendable.
I am looking through your post and unless I missed it, I can't identify the specific challenge or goal. are you having trouble getting more loans, is it you don't have enough for down payment for next property, are you looking to grow but not too fast? There are different approaches based on your needs; if like me you have enough for down payment but Debt-To-Income DTI was to high (and mortgages are held in personal name) a refi to a commercial loan is a simple process and you don't have to take on anymore debt on that property, if its saving up for down payment is the concern then a cash out refi or HELOC might be a good route for you. If you are purchasing only investment properties consider looking into Associated Bank, Educators and TruStone, because they are local they may have lower down payment requirements. 15-20%
On the Refi Rates there shouldn't be too much change in your repayment amount, if we are only changing percentage on a 150k with 25% down 3%, and we change to 4% and everything else the same mortgage goes up about $60 a month
Additionally, your first duplex in 2020 you have lived there over a year so you can get a low down payment and low interest rate loan on an owner occupied 2-4 unit whether that is through an in house bank/credit union product or the FHA program
Im open to any questions you may have.