I'm trying euphonium again
It should not be a great act to play easier parts on Euphonium after a short time. My son was able to play songs in a one octave range after one or two weeks (in age of 7). My daughter plays also bass clarinet (and cello, clarinet, saxophon) and is able to play so-called "baby parts" without serious practising. Bass clef is a good thing, but all euph parts are also available in treble clef. You can play even your bass clarinet pieces with the Euphonium because the transposing is exactly the same (one octave and a second). Note that written c's and g's are open.
My start in our local band after a few months learning Euphonium together with my son was the following: I sat down, got the sheet music of 2nd Suite by Holst and had to play (including the solos). (Remark: It was IMO not the right way for building a good embouchure to start with such a piece. The higher range has to be improved step-by-step.)
Perhaps it would be possible for you to get the "usual" parts and practising them at home. Although, practising long notes is also very, very good
Jochen
Boosey&Hawkes Imperial with SM4(U-X),
YEP-321 with DW 4AY ...
... and my cello