Check out Superdry t-shirts. They are available in plain colours and are sooo soft.

EDIT: you also might want to try shirts and t-shirts from the Amazon essential brand. Especially their shirts are nice. Not tight, not short and a nice material. E.g. https://www.amazon.com/-/de/dp/B07F26TCYK/ref=cm_sw_r_cp_awdb_imm_4ZRBMN14NZ38RG4JDH11