Floating brackets for upper level support - anyone done this?

I have done what you want to do. Here are some of my suggestions

Check out lower level track before putting up support posts. Check for clearance to the back wall curves with steamers. Then check out clearances of the posts on the lower. Easier to fix now than later.

You will not have access problems with a 12 inch clearance. After you complete the second level, the only access issue should be track cleaning and derailments of the lower level. I have 6 to 7 inch clearance and have no problem with cleaning. derailment access is marginal at 7 inches.

I used dimensional lumber for my posts: 1.5 inches by .75 inches with a base similar to yours. I put them on either side of the two track upper level. Single track upper level, I used one post dead center. Post are spaced about 16 inches apart. Half inch plywood use for platform. Posts are secured the posts from the top side with screws thru the support base. Upper level is secured to the posts for stability. A right angle drill adopter from harbor freight is helpful. All the posts have been moved over the years so you should make them easy to move.

I painted my plywood and supports light gray to simulate concrete. The lower you place the upper level to the lower level, the less the background will be noticed. less scenery to do. wiring to the upper level is attached to the under side of the upper level and routed to the wall. in free standing areas, route the wires on the back side of the posts. paint the wires if desire to hide them better.

Paint the bottom of the plywood before assembly. Nothing ruins scenery like raw plywood. Raw wood on post also.

I've used these blind shelf supports.  They are designed to be set into  drilled holes in the shelf material, but 3/4 stock will be too thin especially if using on wide shelves.  You could rest the shelf on top of the bracket and use small pipe clips to secure the shelf.

MDF will tend to sag between supports over time.  I'd use the 3/4 ply

https://www.rockler.com/i-semb...-blind-shelf-support

I installed 2 floating shelves for a customer. They were 2" thick and 10" wide and 8 foot long. i used brackets from sheppards brackets. Very sturdy and expensive.  They also have brackets for smaller projects. You would have to google sheppard brackets