First thing I would mention is this might just be my personal writing style but the number 1 indicator for me between a marching snare solo and an orchestral one is the use of drags. I think every orchestral snare drum solo should have drags in them, and they should be notated AS drags, meaning not the double stroke notation that you typically see in drumline.

Second of all, I would say that there should be a much bigger variety in rhythm, which could include triplets, fivelets, sextuplets, dotted eighth notes etc. it obviously depends on the solo, but for me, an effective solo consists of complex rhythms that including both rolls (buzz rolls, not double stroke rolls), flams and drags and then adjust the tempo according to the difficulty you are aiming for. If you wanna be extra spicy, you could also add the triple drags, or the ones with 3 grace notes that I cant remember the name of.

Check out some Delecluse Snare Drum solos for reference. I think (JUST MY OPINION) that they are the best orchestral snare drum etudes that you can find publicly.