Hey Gavin,
I think what you are doing is setting a great foundation. As long as you like your job and/or industry and could do it for the next 15 years, what you are doing would work perfect for your plan.
As you gain experience, maybe you can move into more units per purchase or start to BRRRR. I'm sure you will gain some interest from potential investors then maybe they can supply you with the down payments and you can pay them back over time and you could buy more per year.
From what I know, business is a slow game and from what I hear, successful Real Estate Investing can be boring. Your investing plan sounds "boring" but in 15 years of this strategy you will have millions in assets that cash flow you to not need a job, and equity built up for continued growth or expansion.
Is there a way that you could speed it up to buy 3-4 per year? Maybe that challenge would make the investing journey seem more "fun" but you are in a position that sooooo many people dream of. You could start BRRRRing houses and maybe that would allow you to buy more and make more cash flow.
As for the paying off the mortgages, I would make you minimum payments monthly and when rates go down in years, you could refinance but pay the same amount and that will help you to pay it off sooner while you are left in the same Cash flow position.
Keep it up!