Is the thick mineral wool batt actually something a regular person can buy?

Before you buy anything, measure the actual foam depth on those walls yourself. Closed cell sprays uneven, and if you relly have a full 2 inches in a 2x6 cavity, you've only got about 3.5 inches left to fill. That's exactly what the big box batts are cut for, and R-15 in the remaining space on top of 2 inches of closed cell is a solid wall for a bonus room. The foam is your vapor control layer, so the batt on top is just there for R value and it doesn't need to be anything fancy. On the stacking question, yes, it works fine and people do it all the time. Mineral wool is stiff enought to friction fit without slumping. Run the 3.5 inch batts first, then cut strips from a second batt to fill whatever gap remains. The trick is cutting the strips a hair oversized and pressing them in by hand as you slide them into place, not wedging a full batt in and crushing it. Offset the seams between layers like brickwork so you don't line up a continuous gap. A bread knife or serrated drywall saw cuts the stuff easy.

The thick commercial batts do exist, but they're sold through distributors, not retail shelves. Skip the online quote forms entirely and call a local insulation supply house or a contractor-oriented lumberyard and just ask for mineral wool batts in the depth you need. A lot of them will sell you a bundle or two without blinking. If they won't, ask a local insulation crew to order it on thier account and pay them for it, which works out cheaper than retail anyway.

One caution on doubling up: if two layers won't fit without squeezing, use one layer plus trimmed strips instead. Compressed mineral wool loses R value fast and you defeat the purpose of the snug fit.

Did the two layer thing in my garage attic last fall and it went fine. One tip that saved me some headache: cut your strips on a piece of scrap plywood laid flat, with a straightedge clamped down. Trying to slice it freehand against your knee gives you wavy edges and those little gaps add up. Also wear gloves and a long sleeve shirt, the fibers get in your skin and itch like crazy for hours. I ordered mine thru a local supply yard over the phone, took two minutes and they had bundles on a pallet the next morning, cash and carry. No forms, no waiting.

I ran into this on my workshop build last year. Turns out the counter guy at a smaller hardware store could special order the thick batts even though the website showed nothing. Cost me way less than I figured too. So don't write off local stores just cause the website looks empty, sometimes they've got a catalog they don't bother listing. Also FWIW mineral wool's pretty forgiving to work with, way more than fiberglass. If a cut's a half inch off you can usally nudge it tight with no real penalty.

One thing to add about filling those cavities, the 2x6 designation is nominal so the actual bay depth is more like 5.5 inches once the wall is framed, which matters when you are eyeballing how much room is left after the foam. Also, mineral wool batts split cleanly right down the middle by hand, so when you hit wiring or an electrical box you can peel half the thickness off the back and push it in behind the cable instead of smashing the whole batt around it, then patch the front with a scrap. I did that on every box in my basement project and the fit came out neater than anything I managed with fiberglass.

Reading this whole thread with great interest because I'm about to do my garage ceiling and was leaning toward the fluffy pink rolls just since that's what's always on the shelf. Never realized the rocky kind splits in half by hand like that, that alone might sell me. So is it heavier to carry up a ladder than the pink stuff, or about the same? Also does it matter which way the paper-ish facing goes, if there even is one?

Update: measured the foam in a handful of bays like someone suggested and it's uneven, closer to an inch and a half in spots. Good call. Also called a local lumberyard and they can order the thick batts, so sourcing is solved. Still unsure how to handle the shallow spots though. Trim the batts to follow the foam, or fill the low areas with offcuts? Anyone dealt with wonky foam like this?