Brass Band players usually read Treble Clef. It works OK, and for doubling it is great. I ran into issues recently with a Low Brass Group - my Bass Clef reading sucks, and using Slide Trombone it just messed up badly. Trombone reads Concert in Bass Clef, and Bb in Treble - so can mess up badly if you think the pitch and note in your head. I now keep all Valved instruments for Treble or Tenor Clef, and have dedicated the Slide Trombone to Bass Clef. It seems to work OK, but I also know that if someone gives me a Treble Clef part to play on Slide Trom, all my effort on re-programming my Brain will be gone in 5 minutes. My advice is to learn Bass Clef with a Slide Trom, and not to use the trumpet positions as a starting point. Although if you just want to double in Treble Clef, then it is an easy way to learn. I had my first Slide Trom gig about 2 weeks ago, a 3 hour Garden Party with Big Band. It is becoming comfortable to read and play Slide Trom in Bass Clef - a slow journey for me. BTW - For the OP a Slide Trumpet may be an option??? And some fun.