3rd time in 3 years?

This is an unpopular idea but the reason the caulk between a tile wall and the tub gets moldy/fails prematurely is because the tile grout is porous and the water travels down [osmosis] until it is trapped by caulk at the tub. The constant wet spot allows mold to colonize.

It has become "common sense" to caulk that interface but if you were to remove the caulk the mold would not form.

OTOH to use nothing there requires careful waterproofing at the level behind the tile. A properly constructed shower should be functional/waterproof before any tile is installed