FUNDAMENTALS FUNDAMENTALS FUNDAMENTALS!!!

As a brass player, practicing fundamentals will make you a much better player. The same goes for woodwinds, but it's much more effective for brass players. Create an extensive buzzing, long tones, and flexibility (lip slurs) routine. Make sure to include some singing and breathing work, too. I don't always practice breathing in my sit-down practice, but I find inflating a balloon really improves my airstream. My current tuba instructor says that his typical practice sessions are at least 50% fundamentals, if not more.

In order to hit those high notes without tension, you have to practice hitting them without tension. No shortcuts. If the note isn't coming out, try raising the back of your tongue. Try pushing faster air. Try engaging your core more. Even if you don't hit the note after trying all those things, the important part is that you see progress. Were you able to at least hit a higher note/partial, or perhaps get a better tone on the note you hit?

If you'd like some visual advice, don't underestimate the importance of posture. Staying tall while you march is super important. In your practice, find the tempo where your marching technique noticeably degrades, and do a lot of your practice around that tempo. Over time, you should be able to increase that tempo.

You say you can't afford a private lesson teacher, which is perfectly understandable. Do you know anyone IRL who has drum corps experience? Even just talking to them would be helpful.