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?
That's why I came to the community. I feel like I'm a reasonable and half-way experienced DM at this point. The player is aware cantrips are unlimited use. The player is a very intelligent individual. I've had the conversations about spell use. At its core, I genuinely think the player is attracted to the "cool factor" and "aesthetic" of playing a caster but doesn't actually want to engage with any of the mechanics. I can remind the player about spells and that reminder will last for a combat, sometimes less. I feel like I've done everything I know to do aside from straight banning the player from playing full casters or queuing them to cast spells every-other turn, I'm at a loss.
You'll have to start killing their character more often. They want to do front line tactics, they can take front line damage, no softballs. Oh, 1d10 kills you in 3 hits, and the mobs chase you because you're squishy? Damn, maybe get some AC and constitution, or attack from a distance.
I go back and forth on this. I feel like I'm enabling these choices by pulling punches. But it feels excessively anti-fun to just kill them and be like "sorry lol be better". I don't think I have the heart to just murder characters all the time.
Do you have the chance to sit the whole group down together and tank about that?
As in, tell them what you told us - you can't dm as you normally would and have to keepepulling punches because he keeps endangering the group and you dont want to punish the group for his bad choices. Remind them that you are supposed to have fun too and dming like you're walking on eggshells is killing yours.
I'm suggesting bringing it up with the whole group because it sounds like you brought it up with the player alone already. Or maybe you have focused too much on how they could play better instevd of how it negatively affects the group. Maybe they need to realize that even if this is fun for them, it's not for you - and maybe not for the others as well.
IMHO, this is something that should be talked trough with the group. Getting less lenient and not weakening your encounters is risking the wole group's fun, so it should be discussed with the group.
Enabling choices per se is a good thing, but if you have to rely on someone for life and death who knows how to shoot a gun, owns it, brings it and yet only uses a knife in the gunfight, it's gonna get you killed. And this should happen. Just make sure everybody knows about this.
Maybe this will spark discussion about why he makes these characters and why he plays them like this. Maybe it can help find something he enjoys. If not, the group is now prepared to die more often and you don't have to pull punches.