As far as munis go, I keep mine in closed end funds. I have been in IQI and NEA since 2015. They are currently yielding 4.77% and 4.56%, fed tax exempt. These both held up very well during the COVID dip. They have slowly bled off yield (I originally got almost 6% yield) as the shortest during bonds in the fund mature and have to be replaced with lower yielding debt. But the NAV has climbed. If you are holding individual bonds, switching to these would increase your diversification and yield to the point you are benefitting from the mortgage leverage.
That said, I paid off my mortgage before putting a penny in the market (save what I could get a company 401k match on). I never regretted that mathematically sub-optimal decision. It sure feels good to own your home free and clear. I sleep well at night. And it gave me the courage to take on some riskier investments and learn options trading. I never would have made the sort of early returns I did without the mortgage paid off b/c I never would have bet the "house money". What I'm saying is, maximizing your lifetime earnings is not always the best choice, especially for a mustachian. Do what maximizes your lifetime happiness.