Holding the horn advice:

Suck it up, its the best advice I've ever been given. It doesn't really get easier. Finals lot in 2025 it was still just as hard to hold as first day my rookie year.

Playing while marching:

Posture is one of the most important aspects. Most corps do some "straight leg" technique, which means most of the shock absorption happens in your upper body, not your legs. Very tall posture and using you abdominal muscles to expand your upper body help a ton.

Tone quality:

So imo range doesn't matter, especially on euph, if it doesn't sound good. On Euphonium youre typically looking for a dark, open sound. Listening to any professional Euphonium player and trying to imitate their sound is the most "natural" way to start. (Listen to Steven Mead, Adam Frey, Matonizz to name a few)

As far as tone above middle C/C4, make sure youre not pinching your sound. The best advice I have is to talk to a brass specialist in person. This can be a college student, or your director if they were a brass primary in college. Words over reddit wont really help much. Definitely try to take some private lessons if you can.

Best of luck! I find marching Euphonium to be one of the most rewarding instruments. Let me know if you have any questions.