It seems like you have a stable, long-term plan for FI (or at least retirement), but you're window-shopping real estate and eyeballing that pile of cash with an itchy trigger finger.
You're looking unevenly at the best and worst case scenarios. Best case, great tenant and more retirement savings from profits! Yay! Worst case, no tenant and lowered retirement savings. Boo!
But we all know that's not the worst case scenario. Bad tenants, major repairs, liens... Worst case, you've lost that pile of cash and cannot afford to plug the hole your income is pouring out of.
You're also underestimating the benefit of staying on the present course. You are already landlords and have steady retirement contributions.
What's so bad about using the pile of cash to pay off the student loan? That's the reason you saved the money, right?
If it makes you feel better, don't pay it all off at once - keep some cash liquid to make the window-shopping more fun.