It is an oddity in 5e that cantrips grow in power whereas leveled spells don't. That said, before 11th level your 1st & 2nd level slots should still be more damaging than cantrips.
- A 2d10 Firebolt with a 70% chance to hit will deal an expected 7.7 points of damage.
- 3 Scorching Rays with 70% chance to hit will deal an expected 14.7 points of damage.
- 3 Magic Missiles with 100% chance to hit will deal an expected 10.5 points of damage.
However, I believe there's an item that allows you to adds your Int mod to cantrip damage..?? This would push cantrips beyond 1st level spells, and is probably an argument for it's modification or removal.
As @Kendaric says, Enemy stats (and thus Saving Throws) are buffed in BG3 and you can't benefit from high ground while using Saving Throw spells, so attack roll spells are better. However, there aren't many attack-roll damaging spells at higher level, so you're forced to rely on ST spells.
As for rolls, I'd be fairly surprised if there was different rng for cantrips vs non-cantrips. Like, I'm no fan of Larian's historical implementation of rng (Karmic or non-Karmic), but that'd be too far for even them. I suppose the rng on XdX damage could be coded such that the same number isrolled more often, but this would result in both extremely low and extremely high totals...
