Howdy.
IMO, you are overthinking this ... not that I don't do that all the time, at least WRT my Guzzi harem.
First, not sure what you mean by "hard cases." Lots of variances out there, including, I imagine, in composition.
If, however, OEM, I presume pretty much what I have on my Norge.
If so, I have used most anything and everything along its 85K miles to keep them looking nice.
While I have been a sucker for most every new "magic, breakthrough" product that comes onto the market -- "Operators are standing by ... but, wait, there's more!
-- my go-to for most cleaning these days on all moto-tupperware is ... yes, I'm serious -- Pledge or similar spray wax.
As for your "I keep washing them with car wash detergent/wax, so surely they'll dry out eventually," I don't wash my motorcycles anymore or, at least, very rarely and
VERY carefully.
One $tranded at the $ide of the road epi$ode far from home as a result of a corroded ignition $witch was enough to change my ways.
And, "drying out?" Unless you are working with leather than some form of composite as are most hard cases, I think we'll find out if Greta is right before those dry out, at which point it won't matter!
Anyway, bags are bags, and removable, but I still use "real water" less than I once did.
If just back from a road trip and covered with grime, I'll remove them, clean off the worst of the muck with a sponge and warm water (and sometimes more), then Pledge all over, meaning shiny roadside and unpainted backside.
If doing a midwinter special cleaning, I'll top off with one of my splurges of the latest and greatest sealant.
Oh ... just remembered apropos cleaning vice the protection piece. For the last few months, I've been using this product:
Waterless Carwash. So far, so good. Won't remove dried hunks of mud from the bottomside, but sure works (for me) on lesser cleaning needs.
Enough. Must go, anyway, as Kathi, my Perfect Pillion & Polish Princess tells me that she wants me to make her breakfast.
Best,
Bill
P.S. Before I start on those eggs, sausage, and biscuits, I'll add what I've said elsewhere here before, i.e., that, IMO, your B11 is one of the best all-around Guzzis to roll out of Mandello's red gate. With bags and w/s, it has all the virtues of a Norge without the latter's tupperware challenges.