I think seperates is a good flexible solution, sage grinders have good resale (are burrs expensive)
I've not seen any analysis that says small pf is detrimental .. there are enough supply of tampers/baskets/bottomless for 54's, maybe you can't get 21g baskets, but I've never seen the need for that with a 58 , with coffee prices.
Hard to know to what extent brexit, or covid is responsible for grinders price hikes, but Mignons I had contemplated changing my rocky for that were ~£200 are now nearer £400, over 3-4years