There’s really no right answer here and I don’t think it’s something that we can work through without that player involved in the conversation. It’s not that they don’t know better, it’s not that you haven’t helped them, it’s not that you haven’t made suggestions, and they’ve been doing this for 3 YEARS??? I’m sorry, but this is above my pay grade. I am almost certain there is some detail that I’m missing because this makes zero sense. I have played with veterans of all walks and ages, new players who are 8 years old, new players that are 60 years old, and everywhere in between. It just doesn’t make sense unless there’s more to it.
Sit down with the player again. Ask why they don’t use cantrips. Leave the leveled spells aside for now (saving them forever is a problem, but an understandable one). Continue to remind them every combat, every turn, every time they take out their dagger. I know you said your group doesn’t know the rules well, so maybe it’s time to learn (3 YEARS???). Cantrips and weapons work exactly the same, so I don’t know how “not wanting to engage with the mechanics” has anything to with it. There’s something going on and I can’t be sure what it is without talking to this player themselves
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?