Fellow OTP Garen here (almost 400K mastery points, and more 200 games in Ranked with him this season).
Panth was never really a problem for me. I start cloth+4 pots, and he usually is OOM before I'm out of HP. Use your pots early, BEFORE falling below 40% HP, so his auto-crit doesn't proc: you save a lot of HPs by avoiding it.
Same for Thunderlord, you should be able to avoid TLD procs unless he actually uses his stun to keep you in place. If not, you're playing too aggressive/too greedy for CS. And with good reflex (easier said than done, I know), if you can trigger your spin+W when you see him jump on you, you're dealing free damage and you don't care about the stun. aa+Q reset after the end of E to proc Grasp, then back.
However, someone below suggested 3 points in W, it seems overkill to me. Maybe an 2nd point earlier than usual, but not more.