Warlock eldritch evocations for sneak attack

Was working on a either Rogue subclass that gives access to warlock evocations or a warlock with sneak attack spells.

I settled on a warlock evocations called 'eldritch ambush' that can apply sneak attack roll to non Eldritch blast cantrips/spells. It would require the normal sneak attack req. of having an ally near by or would normally roll with advange. How would you make the damage scale with out being broken/weak I initially thought 1d6 per per charisma bonus. maxing out 5d6 + (1-4)d6 for say mind sliver.. at max level is will average 36-ish damage to one creature. which at max cantrip at level 17 seems a little week, but would about equal what a rogue gets at the same level, without the utility that rogues get in swapping out d6 for status effects. i would like for it to apply to any spell cast but can see that being a bit much, so have it forced to choose a cantrip upon selection does seem more in theme with the other lock evocations.

More information: would be for a great old one patron. Originally was 1 rogue/2 lock at start for sneak and proficiency. I know EB is iconic and i think i would still take it, but love that i can apply evocations now to any warlock cantrip and love the flavor mind sliver adds for a GOO lock.