'Thick and stiff' (lol) for demanding lead parts - something like celluloid/tortex etc. 1mm. For strumming I prefer more yielding material like nylon, around 0.88 mm.

I like thicker stiffer for electrics and agree that strumming sometimes demands thinner/flexible pick. But if I only had one pick, it would be the thick/stiff one. I actually like the finger picking styles of PPM from the 60''s so don't usually use a pick with the acoustic guitar.

I used to use a rock and found strumming anything funky totally wasn't working. Then found control on leads with a thin pick was a pain too. settled on an ernie ball, nylon, dark blue medium. gives me best of both worlds and make me not sound quite so bad, lol ! Cover your ears they're about to undergo an assault!

I'm a bassist.
I don't understand about Thin and Flexible stuffs.
😑