Holy shit, I hope you've atleast been switching the brush head every 3 months.

1. Asket (note: not H&M's greenwashing brand Arket) makes my favourite t-shirts. They have a slightly thicker standard version and a thinner, more breathable, lightweight version. Can strongly recommend both, although the lightweight version is naturally less durable than the standard one.
2. What helped me was switching out my aluminium-based deodorant for an aluminium-free one. Just check the ingredient lists, if there is aluminum/aluminium mentioned, it's a no-go. Haven't gotten a white or yellow stain in a shirt's armpit since.