The standing recommendation is to make your own copy of sky130.lib.spice and define your own ffll, sshh, etc. corners, including the appropriate mix of files from the transistor corners and the r+c corners. One might worry about creating corners that will never physically occur with that strategy. You could end up doing battle with an issue your sims show in the fshl corner, say, that will never manifest in hardware, but then, you could just chalk that up to guard-banding. So, yeah, splitting all.spice is a possibility. Or defining your own mos.spice and r+c.spice. Or the mixed corners could find their way into all.spice, which is what I would like to see. That would help with making regression testbenches that are shareable.