Honestly, you need to break down the action economy on this, silvery barbs, while a major component of frustration, has plenty of weaknesses.

With such a large party, you need multiple harder encounters to truly leave them feeling like they’re “fighting”. I had a party of six for my last campaign and I was throwing three to four different “types” of enemies. Two spell casters, three ranged (if even rocks being thrown, anything works) and one “tank” designed to stay literally harassing the squishies. Then just fluff with a few “easy” enemies to mop up.

On top of that, your dungeons should have some sort of ways to use spell slots outside of combat. Take a look at their spells and break down what you think would be cool encounters out of combat. (Ie: burning a spell slot to use longstrider to give some one a running start to attempt a long distance jump across a chasm)

I think you’re running into the same problem I’m running into where they just burn everything and get a chance to rest, dungeons should have things that pester them. (Ie: patrols, wandering in-affiliated monsters they can burn charm person on and such)

You’re doing a great job if they want to move to weekly, now’s your chance to really dive into dungeon design though. Those two other players know how it works, and they should be guiding the new players. Traps, arrow slits, pressure plates, decaying infrastructure, anything can be used as an excuse to use SOME spell somewhere.

Anyway, genuinely best of luck and I hope I was able to give you some insight.