First of all I would like to start by saying new featured/event combat modes that are not a grind in nature are highly advised. With that said, I think I have a couple ideas that may make the game as is feel less grindy.
The first would be to do two things to invasion. Cut the number of days down to 3 or 4 days because the top spots really aren’t not changing excessively through the week until everyone does their big push in the last 24 hours. Also take away the ability to use multiple shields to multiply damage and replace it with the option to use multiple shields to multiply your points earned instead. This would remove the grind and add a strategic element to the final days push expecially if you increase the final hit bonus on the bosses.
Additionally although they require more programming, you can add a very rare drop only 5x and/or 3x damage bonus buff item to replace the lost feature if your feeling frisky and maybe a fill energy item that isn’t so rare that acts just like the revive but for energy instead of hp if you’re feeling extra frisky. Or mix up the whole thing by giving the boss and their team different buffs/debuffs like col and arena rules.
Second would be always make sure arena and col groups have only ever have 14 or 21 teams. Doing 4 attacks a day when the groups get too big feels a bit too grundy but 2-3 attacks a day feels about right to me.