I'm debating about doing this for one of my rentals. My initial strategy was to do a BRRR since I was adding equity after significant fixups, but now I'm not sure if leveraging it would be the best.
The commonly-held wisdom I've heard is that a mortgaged property is ok as long as you have a plan to pay it off before FIRE. Following this, you could pull the money out, put it in a more liquid vehicle like an index fund and start collecting the cashflow into your fund. The balance of your fund goes up and your mortgage balance goes down. Once your index fund has a bigger balance than your mortgage balance you can technically FIRE by pulling the money out and pay the mortgage off. Or, you can keep the mortgage and keep the fund and can technically say "I'm debt free on this property, maybe, cross my fingers!" and let it ride.
But, this whole "payoff" strategy presupposes that you only have rentals. If you have other cashflowing things in your portfolio that aren't debt based, like notes, that helps stabilize your income against things like vacancy, but also further complicates things. The more moving parts in your portfolio you have, the harder it'll be to even say with certainty that you can FIRE.
HTH