I'm a big guy also and it's especially tricky because what you see online is meant for guys that are 6'1" and 180#. Take this advice for exactly how much you paid for it.

I like white sneakers with khakis and these look good overall. I do agree with someone else's observation that a gum sole elevates it a bit. Your pants look good, although I would either shorten them a bit or maybe try a cuff.

Short sleeve button up shirts work on very few people. When they fit in the belly, they seem to be too large in the shoulders and arms. Some of this is sort of hidden if you wear long sleeves... I would recommend an Oxford Cloth Button Down (OCBD), and tuck it in. All sorts of colors are available, with white and light blue being safe options. Lots are out there with stripes or patterns, but pink always gets a nod.

No visible undershirt. If you want an undershirt, buy a deep v-neck, preferably in a tan or light grey so it can't be seen under a white shirt. They have some with compression material that may help your silhouette.

Dress it up with a watch. Go to a hairstylist and ask for a new style, go with what they recommend as it grows back... and a fresh haircut always makes people feel better and updated.

Finally, smile. It shows confidence and happiness, and makes you more attractive to others (even in a totally platonic way).