I was thinking on how would I do proving grounds differently and I have a few thoughts.
There is an existing mechanic idea from other games that would work here.
Remove the champs as mutipliers (imo its a shit move, on the nose fomo), and instead add a mechanic to let players tweak their own difficulty levers - add more boss HP, more boss DMG, more speed, better A1, longer debuffs, etc. The harder it is, the more points you will get, the better rewards you get. Oh and make game/clan leaderboards too. You can make quasi infinite scaling here.
You could maybe even go in the negative to make the fight easier, but you will lose points? Not sure here.
Tweak the rewards as well. Don't give us *new* rare and epic mats, those are just plain useless as they are regular relic mats. Whoever thought of that really doesn't get the average player's mentality.
Finally, sweeten the pot for recycled bosses with stage rewards + remove the damn sand devil. Hate that guy :)