If making a remove all toys and equipping the best ones button is too complicated--maybe you just could deactivate all toy effects (for both player and opponent -_-) during colosseum battles.
That would be a complex "rebalancing", or else the new players couldn't advance and all the fights except against the event monsters whould be very slow.
Rebalancing is a thing what we need the least. They had already tried it some 2 years ago, then had to revert everything. But the numbers was so different that developing your team in those months proved to be a long lasting disadvantage.