Have a look at Seagale - their action merino range fits a slim physique nicely
Fresh clean tees, they make shirts for everyone. I'm 6'5" and used to be hella skinny and their shirts always fit me well.
https://rapanuiclothing.com/ make nice plain tshirts and are quite big on sustainability. I am 180cm/77kg, wear size L and they fit nice not baggy at all and wash well
Curious about this as well.
OP, where are you located?
I don't know how crafty you are, but you might consider fitting your t shirts yourself. Tock customs did a great video on that exact topic, and you really only need a sewing machine and scissors.
When shopping look for 'fitted' tees. They'll have a narrower trunk. Also I like Next Level tees, they're cheap and good quality.
Idk about ethical production and all that, but the 32 Cool t-shirts have a nice fitted cut. The best part is you can get a 3 pack at Costco for like $12. 50€ for a T-shirt? Are you crazy?
Male Fashion Advice