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This is my third year playing the euphonium. I can play from low F (three ledger lines below the staff) up to high E (three ledger lines above the staff) on my 4-valve euphonium, and I can do single tonguing at 220 BPM. I’ll probably keep playing for a long time—who knows?

Could anyone recommend some methods, etude books, or study materials to help me keep improving my playing technique? Thank you very much!

It sounds like you've already been working scientifically and have developed solid technical skills. I'd recommend working on the opposite for a while: having an emotional connection to what you're playing, having fun, creating beauty.

You've been working it, which is great. Now's the time to start playing it.

Take lessons with a pro player ....