You need legs and some stretcher bars to create a table. It has to be supported down to the floor. The steel braces work with the principle of a cantilever. That means that you have to have 2/3 of the weight of the top on top of cabinets, and only 1/3 supported by the cantilever. If the weight distribution isn't correct, it can tip, regardless of "support" underneath. Also, 20" is WAY overkill for seating depth, so one other option would be to reduce it back to the standard 15" that is recommended for counter height.

The 20" overhang does have about 30-36" of countertop on the same slab supported by the existing cabinets. We want to extend to 20" to gain more countertop space for our kitchen, so it wouldn't be for a seating area. I wouldn't be against using legs on the end of the overhang.

If you don't want to add corbels, you can use the metal brackets or do as GreenDesigns suggests with table legs and a stretcher. With a 20" overhang and the metal brackets, I would go with the ones that are 1/2" thick, not the thinner ones. However, what would you secure the brackets to? Is there a half wall behind the cabinets to secure them to? You do need something solid to mount them to.

If you don't want seating underneath, the best thing would be to add more cabinets. That way you get more storage as well as more counter space and everything stays supported. Just check with your aisle measurements. You need to be around 42" for a single cook, and 48" if you want multiple cooks to work in the space.

I think you are at the maximum length whereby you can use metal bars and not resort to corbels.

We had a 2cm quartz peninsula with 12.5" overhang. We used 3/4" plywood support underneath, then inserted 2" x .375" steel bars into grooves in the plywood that the contractor made. That way the countertop laid down flush over the support. The bars went under the full length of the countertop.

If you look at the Pental quartz install guide, they should say this approach is recommended for overhangs up to 20". For other manufacturers its probably similar.