There is no downside to "signing up" for PSLF. It's a form you fill out every year. Most people won't qualify for PSLF, since they either go into practice after training and their salaries disqualifies them from PAYE/IDR. The only people who may qualify are idiots like myself, who goes through residency, then fellowship, then super fellowship, then research. Even with this route, I'll still be at 8 years, and may need 2 more years of instructorship to qualify for PSLF.
Forbearance is never a good idea. PAYE will give you the lowest monthly payments. If you spend 10% of your salary on PAYE, 10-20% on taxes (effective tax rate, higher in CA), ~10% on 401k/403b at least to make sure you get the match, that should leave you at least 60-65% of your salary for living expenses. Subtract 33% for rent in CA, you should still have more than enough money to get by at least intern year. After that, take Step 3 and start moonlighting.