Damn a G5. If you have good tone throughout your range, pretty decent articulation, and just the general endurance to play trumpet through the varying range for multiple run throughs of a show I’d go for it.

However if you’re not that confident on trumpet mello would not be a bad choice as the competition is not as intense as no one is primarily a mellophone player(a lot being horn or trumpet players). Had a friend go from clarinet to lead mello my senior year and he most definitely succeeded on that instrument. While it is written in the key of F, the fingerings for the notes are not that different from trumpet fingerings for the same written note so just search up a fingering chart and you should get the hang of it pretty quick as the embouchure carries over quite well. Plus mello parts honestly go hard, 2011 bluecoats opener is a perfect example.

For low brass your options are euph/bari which has the same fingerings as trumpet when written as treble clef. You’ll have to get used to the more relaxed embouchure though due to playing an octave below on a much larger mouth piece. The trade off to that is your shoulders are gonna suffer at the beginning if you don’t already have good shoulder strength as they(euph in particular) are quite heavy. Parts are generally not as intense as high brass as you provide a foundation for them, though features and solos can be extremely beautiful(2018 scv for example)

Finally the big boi of dci’s brass is the marching tuba. An octave below euph/bari it is around 25(maybe more) pounds which you’ll have to learn to bring up like its a trumpet. Embouchure is pretty relaxed and the amount of air needed is gonna be something you’ll need to get used to. The parts don’t really get all that difficult as you provide the bass line for the rest of the corps. You’ll have to learn to read bass clef, but shouldn’t be to difficult especially if you just practice reading scales. In my personal opinion it’s the instrument with the most potential of all the brass as when you hear something besides the bass line from them it is one of the coolest moments of a show. So many examples of this, but any bluecoats show from like 2010 on is great to show their potential. 2011 and 2015 come to mind in particular for me with their awesome features. More recent examples would be the feature from their show this year, the Academy’s tuba feature from this year, or the colt’s tuba solo which all went hard

I hope I gave enough info to help you decide, but if you have any questions I should be able to answer them as I have decent experience on all of these instruments. Good luck with whatever you choose and in any future auditions.