I guess I could try making profiles with multiple mix options and then test them all at the gig to see which is better.
That sounds pretty painful and cumbersome - having to replace the entire profile in your performances just to change the mic balance.
Maybe easier to separate profiling and speaker miking: do one DI profile of your amp without speaker and create different Impulse Response mixes (with varying levels of the 121) to try with your profile. Now at soundcheck just switch IRs with the same profile until you're happy.
You can create your own IRs of your own speaker cabinet, if getting "that" sound is important to you, but it's a bit cumbersome (https://www.youtube.com/watch?v=1E3VSKjzdgU)
Alternatively, you can use some ready-made IRs and blend them together, e.g. with MixIR.
But of course, you may want to stay purely within the Kemper. In that case you could create (merged) profiles with different levels of the 121, and then create Cabinet presets from them (only the Cabinet block). You can then apply these Cabinet presets to any existing profile. So no need to change all your existing (painstakingly tweaked) profiles - simply change the Cab block. So you may have Cab block presets with the 121 at -3, -6, -8, -10, -12, ... Simply swap them out until the sound works with the band - then paste that Cab preset to all your other profiles and performances...
Using Cab presets across all your sounds (even different amp profiles) is also a good way to avoid acoustic vertigo, giving your sounds a common flavor...