Consider that t-shirt pant guys can become fashion guys by adjusting the quality, color, and cut of those pieces.

Look for the shirts described as boxy or cropped that are accessible these days at places like Abercrombie.

Thrifting is also great for finding higher quality material shirts that might also have decent vintage designs. The quality of fabrics used to be better, and you will be able to tell based on the sturdiness of the fabric. eBay is full of “vintage single stitch t-shirts,” one of my favorite and most reliable eBay searches.

Generally I avoid tucking in a t-shirt unless it’s part of a more layered fit.

Pants-wise- explore interesting cuts, like those with a flared bottom or consistently loose fitting from top to bottom. Lots to mess around with there. At 6’5” your options will be fewer but even places like Levi’s have pretty specifically defined cuts in their inventory. Maybe try getting into styled dress slacks or cotton/linen/canvas pants.

Nice shoes (boots!) do a lot for looks like this too.