Alright, thanks. I thought the effective caster level for spell slots in the multicasting section would apply, but looks like I was wrong.
Interesting. Is there a book reference for this or is this just deductive reasoning?
If a character has levels in more than one class, do the character’s cantrips scale with character level or with the level in a spellcasting class?
Cantrips scale with character level. For example, a barbarian 2 / cleric 3 casts sacred flame as a 5th-level character.
It does make a high charisma character dipping 2-3 levels into Warlock to get Eldritch Blast, Agonising Blast and Hex to be an effective base for a blasty build.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.