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
Me: “Is this Big O?”
It is.
I always loved the mystery that this show managed to capture when I was a kid. Especially catching an episode here or there on cable TV. It often meant missing key plot points or not understanding how everything fit together. Watched it through as an adult and I still really like it. I won’t spoil the ending, but it’s one of the most ridiculous things I’ve seen. Not that it’s over the top, but the absurdity of it is something I still reference to this day.
Also the phone ringing to tell you the name of the next episode is burned into my brain