12" is the limit for overhangs, but check with your installer, they may say you're Ok. ( if there's going to be a lot of "leaning" on them, I'd go for the brackets) We've installed metal angle brackets on our projects that are pretty minimal visually. You can probably search on Houzz to find some or build,.com.

Our overhang is 16. They installed these metal bars underneath that extend deep into the cabinetry. Not exactly done much but they figured it out for us. I definitely wouldn’t stand on it( although my husband climbed up on that area to help clean the lights… I panicked for sure… but it withstood it). I wish I could tell you how long they were, but we didn’t measure them before the countertop went on.

Cantilever maximums are determined by slab thickness and you haven't said how thick your tops are. The Natural Stone Institute allows 3cm natural stone to be cantilevered up to 10" with no additional support. If your quartzite has a mesh backing I'd use the supports anyway.

I'd let 1/2" x 2" steel tube into the cabinet rails if you've got the room and stop them 4" short of the end of the top. If you're using full overlay doors/drawers, you'll have to rod the top.

Hi Joseph - 3mm quartzite with mesh backing. Yes, full overlay doors, but the overhang is on the "sides" of the cabinets, not the front and back. Cabinets will be installed "back to back". Does this change your answer at all?

Thanks, Lisa

I usually do steel supports on any over hang and for sure with any natural stone with some natural fissures To be safe just do steel and IMO it makes no difference where the overhangs are .