Why not do both?
The truth is there are VERY few people who make a living solely off of marching band related jobs. Most are doing it as a side gig to make some extra money, but mostly because they are passionate about making music. Just because you love a thing doesn't mean it has to be your main source of income.
If you want to pursue music full-time as a player there are litteraly almost zero paid positions as a performer in marching bands. You could go into the military in one of their bands, but those are extremely competitive positions to get. Most performers are making money in other types of situations such as orchestras, studio musicians, etc.
Essentially, unless you are a top musician in the world, marching band specific positions aren't going to be easy accessible.
Computer Science is going to be a much more stable career path, however it is also a lot of hard work before you get to use your creativity to make things that you care about. And it can be less fun at times, but it is highly rewarding to solve problems with things that you create from nothing.
A spot that I ended up really embracing and enjoying is frontend development because it combines a high level of creativity with the need for coding expertise without diving too deep into intense programming.
It would be awesome to see what career path you could find that blends two passions into something that matters to you!
Whatever you do, be the best at that thing and things will work out.
TL;DR: There aren't many high paying positions for marching band specific jobs, but that doesn't mean you can't work as a consultant for suplimental income while your main job is in something more lucrative.
Source: I used to study music in college, but decided I didn't want to teach full-time. I fell into working on design and frontend development and it has been a very rewarding and reliable source of income for my family.