I actually think, drawing inspiration from another person's post on this forum, that the answer is simple.
Introducing casual but maintaining weapon and xp progression has drawn all the sweatys in to smoke the casual players so they can level up stuff easier.
Here is a simple solution. Make Portal and Casual progression SEPARATE from regular MP. You can even do it via a 4th loadout called "Casual/Portal"
Each casual Portal server restricts you to THIS LOADOUT ONLY. Your progression to that is tied directly to when you play these modes only. You will not be able to use these upgrades in the regular MP.
Which should chase away the SWEATYS from playing - because they don't get anything for it.
if they did this... I'd even be willing to start completely from scratch and have 0 unlocks. I don't need it carried over if that is a hang up.
OR
Option 2
Casual unlocks EVERYTHING. That's right. You can literally play with every gun and unlock. And then have it only count for level xp and NOT for weapon unlocks.
I like this less because it takes the feel of progression away... but it might work too.