The mix you use depends on how you treat yours sans. Over water, use gritty mix. Too cold in winter with just a small amount of water, use gritty mix. But if you know what your sans want and can provide it. Just about any mix will do. One person here grows her's out side in regular soil during summer. Also I believe your plants are of the smaller growing 'Futura' group and not the tall Trifascita. And growing very well.

Stush

Thanks Stush! They live inside year-round in a sunny western exposure. My main problem is sunburn if I don't watch carefully. They are pretty resilient, though. I water when they get dry and haven't had an issue with overwatering. (Phew!) I'll leave them be for now. Thanks for the advice.

I would like to add that they grow best when they are warm and in bright diffused light. Yours look very well tended indeed!
Fred

Thanks, Fred! What do you use to fertiluze them?

when I use fertilizer I use Miracle grow liquid, just a dozen drops in a gallon jug.
I don't fertilize but a few times a year (never in winter) I'm still addicted to "potting mixes" sold in the BBS. Someday I'll get Al's mix going, but right now everybody is still in Cactus Mix with extra perlite.