I have the opportunity of joining a group of mixed children and adults all brass. It is a spin off of a successful town brass band. I already play piano grade 5ish. I thought that as I can read music already I just might stand a chance of keeping up with the kids learning wise!!! What are my chances? I have been told most people start with a cornet and move onto other instruments as you progress.
As a teenager I tried to teach myself trumpet, from a tutor book (in the 1950s). It was difficult, and I never made the progress I had hoped. Later, I went on to learn drums from a professional teacher. I played drums for forty years - pubs, clubs, student big bands, Old Time Music Hall, annual pantomimes and all the 'standard' musicals, barn dance bands etc.
Most of the valved brass instruments use similar fingering I think; I once understood trombone to be the easiest to start on - maybe it doesn't need as much 'puff' as trumpet.
So, as you already understand music, I should think it is a reasonable goal.
I add below a link to an excellent county music service:
I'd say start on the cornet if that is an option, as that's generally considered the easiest to get going on. The trombone is definitely not the easiest to start on, and you need a lot more "puff" than a trumpet!
And definitely get a teacher, even if it's not every week - it will make your progress much quicker.