I truly don't like the "spell progression hole" at 10th level for Spellcasting Multiclass Archetypes.
Do you think this modified version would be too much?
(Exper Class Spellcasting and Master Class Spellcasting are 2 levels lower and a new Legendary Class Spellcasting is added).
Spellcasting Class Dedication (Level 2) two level 0 cantrip (+Trained)
Basic Class Spellcasting (Level 4) one level 1 spell slot
Basic Class Spellcasting (Level 6) one level 2 spell slot
Basic Class Spellcasting (Level 8) one level 3 spell slot
Expert Class Spellcasting (Level 10) one level 4 spell slot
Expert Class Spellcasting (Level 12) one level 5 spell slot
Expert Class Spellcasting (Level 14) one level 6 spell slot
Master Class Spellcasting (Level 16) one level 7 spell slot
Master Class Spellcasting (Level 18) one level 8 spell slot
Legendary Class Spellcasting (Level 20) one level 9 spell slot
Multiclassing should not let you be as good as the main class at anything, even at the capstone of level 20. So yes, this does seem too much.
If you're so concerned about there being a hole in spell progression, you could easily delay your archetype entry by picking up the dedication feat at 4th level. This way you'll have a smooth ride from spell levels 0 to 8 without any gaps in the middle.