Pay off mortgage or invest extra money?

[Jacob Compton]

Hello BP! I am interested in thoughts from the REI community on my situations...

I bought my first home in March 2019.
Home Value - 178k
Original Mortgage - 170k
Current Mortgage - 155k
Loan Type - 5% conventional
Loan info - 4.25% 30/yr fixed
Monthly payments - $1477, includes Mortgage, Taxes, insurance, PMI
My brother and fiancee pay the majority of monthly home payments - $1350 total
My monthly payments on home is - $127

I now have all my other debt taken care of and I can begin paying $4000-5000/mo on my mortgage... or I could save that extra income and invest it into a new property. Or I could pay off my mortgage enough to allow me to file for a cancellation of PMI before investing the rest.

My thoughts were that it would be safest/easiest to go ahead and pay off my current mortgage in the next 2-3 years and then I would buy a duplex/triplex and house hack, paying off the mortgage of the new property ASAP while renting my SFH. Rinse and repeat.. in 10 years I can retire living off the income of my rental properties.

Any thoughts on this strategy? It would eliminate the need to find those diamond deals.. I wont need to renovate anything, etc.. I am 32 now so I can still retire in my 40s. I know this strategy is not as quick as leveraging mortgages to purchase new real estate.. I am not gonna have 100s of properties.. but that could be a good thing as well, I dont have to manage a bunch of properties, only 5-10.

Please poke holes in these ideas :)