You have on average 3 years with a kid in HS Marching Band. Yes, hopefully you'll keep kids through their senior year, but truthfully many do end up getting jobs, have their interests change, get burned out, etc. If you're starting 8th grade for marching band that really is 10th grade for you. Unfortunately in a Small 2A I doubt you have any choice but to use them. I think the ideas of leadership roles are good, but for the kind of kid you're talking about you do need to find another carrot.

Maybe some different trips you take? We usually have better retention on trip years for us, that might be a way forward for you. Don't go to the same destination for each trip, give them something different? We did Florida last trip, we're going to Cleveland next year for the next. I'll probably return to the mouse again the next trip. (If I'm still there we might do NYC again the following one)

Reach out to some other 2A directors in your area and see if anyone has some better ideas there. Your problem is not unique to your school, and many have to use their 7th graders and the would put that burnout point even earlier.

Best wishes!