I actually have the same complaint, myself (previous post), your only options after clearing a Delve are (1) Lv.20-50, (2) max level, (3) max level + 10.
This means that if you want to do a pure faction-team run you should be doing it more or less in sync with your optimized-team runs (or just from the outset). Don’t do like I did and grind a Delve up to level 500 only to THEN attempt it with a pure-faction team.
It also means that once you hit Level 500 (with or without a pure faction team) the UI for delve difficulty selection is functionally reduced to TWO buttons, as the third button (max level + 10) is now disabled. Permanently.