In round terms, cashing out the €73k equity and applying it against the PPR mortgage would save around €1,500pa. (It might also allow the OP to switch to a lower mortgage rate due to the lower LTV but that's a detail).
Whereas, all going well, keeping the apartment as a rental might net around €4,500pa, after all expenses and taxes.
Is a projected differential of €3,000 sufficient reward for all the risk and hassle involved with starting a property rental business?