Looking for recommendations: Good quality foldable keyboard

Based on your assumed dimensions, a keyboard of this size does not exist yet. Even if something exists like that it won't be that durable as you describe.
back in the time I used IClever Foldable Keyboard. It was very thin and durable. I think you can get it now for something around $25.

When folded it is thicker and wider, but a bit shorter than the Pro Max.

I quite like the Protoarc XK04. The only gripe I have is that it requires a solid surface beneath it. It's a 65% layout keyboard. There's an FN key to hold down which gives access to the function keys and other features. I find that they didn't make any nutty changes to the layout of the keys to make the fold work. The only exception to this is that they shortened the space bar on the right side so that the fold works. I try to always use my left hand to enter a space. While the keys aren't mechanical, they're quiet and not too cramped. There's a USB C port an small sliding switch along the right side. Unfortunately, it doesn't report its battery status to iOS. Nothing's perfect, but this keyboard is as close as I've found for something its size. It's not too expensive either.

I use the Protoarc XK01 and really like it. It's tiny when folded shut, but unfolds into a full desktop sized keyboard including numpad. I haven't tried it with MacOS if that's what you'd be looking to use it with, but it works great on Windows and iOS. On Windows the function keys (F1-F12) all work as function keys by default, no need to hold the FN key down alongside them.

I haven't tried many other foldable keyboards, but this is surprisingly stable when unfolded. If you're using it on your lap there's a chance it could feel slightly flimsy depending on how it's positioned, but on a desk or flat surface it's absolutely solid.

Similar to Michael's experience above, mine also doesn't show the battery level anywhere, but the battery lasts so long that I hardly even think about it. I've probably had this keyboard for about a year now and only had to charge it once, and that's using it pretty much all day every day. When it does die, it simply charges via USB-C.

With this being foldable, obviously the intention is for it to be used while travelling, but this has actually become my daily keyboard! It might be slightly larger than what you're looking for because of the numpad, but based on my experience with this one I'd say the Protoarc range in general would be good ones to look into.

I have a Logitech MX Keys Mini for Mac. It would be cool if I could get a folding version of that. I did a search for folding 75% keyboards and one keeps coming up but the reviews aren't really that great.