I want a bandoleer like the nylon GI ones but has enough room in the pouches to carry a 30 round magazine instead of the 10 round clips. Do they even exist?
Maybe Armchairtactical make one...
Yes. This.

From my first post.
I have 3 of these.
One is in my "go" locker right now, loaded up with 4x20rd .308 mags. Another is right beside it, loaded with a mix of 30rd and 20rd .223 mags.
Here's a pic of one of 'em filled with M14 mags. The mags that stick out the top slightly are special, non-standard height CMI 25rd .308 mags.

Here's the second one, with my AR-15 with it, filled with AR mags. I deliberately lifted the mags above the top-line of the bandoleer for this pic.

You'll notice that mine lack the white stitching along the bottom that the original one I posted, has. That is because the stitching is designed to be removed if you are going to carry 30rd mags. It pulls right out in about 10 seconds (literally). Otherwise, it perfectly supports 20rd AR mags at the right height.