I've been a DM for about 3 years, and have predominantly run one-shots and short campaigns in DnD5e and PF2e. I have a player who persistently builds primary caster based characters, but then won't do anything in combat but "I stab it with my dagger." They rarely use cantrips, and basically won't cast a leveled spell unless I suggest it immediately before their turn. They seem to enjoy playing despite the fact that they're far too squishy to be a front-line melee character and don't utilize most of their class features. I've talked with them explicitly about how their play style seems to be discordant with the kind of play they want to do, and that maybe next time they should try a paladin/champion or a fighter/rougue subclass with some minor casting. They agreed at the time that sounded like a good idea, but low and behold showed up to the next one-shot with a primary caster, and over 3 hours of play and 3 combats never cast a single spell, including a cantrip.
I enjoy playing with this persons as a whole. They are engaged in the fiction, and are particularly engaged during exploration activities. They tell me they also find combat quite fun, and they are requesting I run a mega dungeon in the near future.
As a general rule, I like to let people play how they have the most fun, but issues have arisen with this play style. Namely, all of my TPKs have been associated with this player charging a squishy character directly up to a significantly stronger villain and continuing to stab it with a dagger until they went down, significantly hindering the party in the action economy and resulting in a TPK. I feel I have to intentionally weaken all of my encounters to keep the party feasible in the face of such mechanically poor combat choices.
What else can I do to help drive this individual towards melee builds, and/or help encourage them to change their play style to better suite the caster classes they choose?
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
My table tends to not metagame at all, even in situations I really wish they would. I think of all the answers I've gotten, this is the most reasonable and actionable answer. Just remind the player more often. I'm gonna have to come up with a good way to not sound like a condescending asshole because this is the only player I'm going to have to do it to. It just sucks cause it's one more thing I've gotta do while running combat. But that's life I guess