I can see where you are coming from but OP assures us that this player knows about games and specifically makes caster characters. This isn’t one sorcerer with a quirk in their backstory about never using their magic, this is multiple characters in a row. I play with new players all the time. Maybe an occasional person will take others’ suggestions as law, but if they do the same thing too many times in a row or force themselves to use the move you recommended when it still doesn’t make sense, you just keep guiding them.
“Don’t forget you have other cantrips too. Using fire bolt was a suggestion. In this fight, you could try using your shocking grasp to get away. Or you could use your magic missile for some guaranteed damage on that heavily-armored hobgoblin. It uses one of your slots, but now seems as good a time as any. They’re no good to you when you’re dead.”
The DM and even the other players should be chiming in with suggestions on other players’ turns. It can get annoying when you know how to play and others are telling you what to do, but if you had a fighter player who just stood in combat and took a disengage action every turn, wouldn’t you eventually speak up and suggest they try a dodge or an attack instead?
Remind them every, single, turn.
“Really? Your dagger? Not your cantrip?”
“Oh yeah, I always forget about those”
If they prefer to use it for thematic or aesthetic reasons, they’ll tell you and the mystery will be solved. Maybe there’s a class that does what they want and you can push them towards it. Or maybe they really are just that forgetful and they just need to be reminded every turn. Consider giving them a character sheet that more obviously shows what they can do. Action cards, spell cards, stuff like that might help too. But ultimately, just don’t let them make a dagger attack. Just stop them and present the better option EVERY TIME. You’ll learn one way or another what they want because they’ll go with it or resist it
And I’m not necessarily saying to find a new group (although it seems like you’ll have no choice since you’re moving). I’m just saying, I cant imagine how you could keep playing with this person and no one at the table is making suggestions on how to play their turns. My players strategize about each others’ turns constantly. It’s a bit meta-gamey, but they’re newer and don’t know all the mechanics well so I never stop them. In fact, I try to help them find the course of action that will make them feel the coolest or the most useful without outright telling them what to do