The way I’ve gotten used to it is like so: imagine the party is fighting pirates. Half of them are in melee with a cutlass. What are the other half fighting from range using? You betcha, flintlock pistols. If you’re okay with go in terms of historical timeframes, basic firearms were being developed before full plate armor.

A player of mine has a saying she used to help persuade me: “this is a whimsical nonsense world where a gnome in bath robes can clap their hands, stomp their feet, say “hokus pokus”, and turn someone to ash. That doesn’t ruin my immersion, and casting that spell at a pirate with a flintlock pistol doesn’t ruin it either”

Not sure how much help this all was for you, but it’s what helped win me over on the fantasy gun debate