With sand and cork and other plastic bits yeah you'd generally get basing sorted before you prime.
Depending on what you wanted to do, it'd probably not be the end of the world to slice or pull the bases off here, get them sorted, and then glue them back on - that would require a relatively flat surface to work though and you'd have to be careful about leaving glue marks etc.
e.g. for snow, you could use cork to raise the parts of the base that would connect to the feet, prime and paint them, stick the mini back on, and then add snow in around wherever you want it.
GW (and a bunch of other companies) also make cool products that work as brush-on basing effects, generally only needing a drybrush and/or some tufts or other small additions afterwards afterwards to make some really nice effects.
Here's a good read of
GW products if you are interested:
https://ageofminiatures.com/texture-paints-review/ My mind immediately went to snow too for contrast; but every mini I've ever painted has a snow base so I am hardly an objective advisor
A sort of grey ashland waste would probably do essentially the same job and if you dotted in some metallic or bone debris you could keep it visually interesting and match some of the colours from the minis themselves.