There is something being rolled out next week that’s a beta thing I believe that will allow you to reorder your reviews by the lowest level items.
That means you should be able to do all of those reviews, up until the last few, without leveling up and unlocking new lessons.
It should also make it a lot easier to burn through them quickly as you’ll know most of the lower level stuff already and so you can blitz through a few hundred really quickly which might make a lower number less overwhelming.