You're being penny wise and pound foolish here. The only way to make such a large shower work correctly is to tear out the old and start from scratch. You will need to upsize the drain size anyway to pass inspections, so it HAS to be done. Then, do a proper preslope, then use Kerdi and a trench drain. You will probably also want to figure in some supplementary heat for this bathroom, as such a large shower will be drafty and cold.

I see mud bed with Kerdi on top as the best choice

If the goal is to convert a current shower drain and a current tub drain into two shower drains, it's a plan that may not work.

Code requirements are different for tub and shower drains, so to convert your tub drain to a shower you'd have to tear out the plumbing anyway to size it to the current standards.

If your current shower is an older shower, the existing drain and branch lines might be undersized for that as well.

So a retrofit might not be code compliant to begin with.

You may not think that a mud pan is a very good design, but if you use a mud pan with a topical membrane, you're getting two things; the most durable and customizable shower base out there through the mud pan, and the best water protection out there with a topical membrane.

Plus, the mud pan with topical membrane will be a helluva lot less expensive, and better performing, than a double-Wedi setup, and much more durable and better performing than anything that TileEZ puts out.

It's your space, but I do encourage you to reconsider your plan. I understand what you're trying to do, which is avoid some ugly up front prep work. But that bit of work up front will result in a wonderful shower at the end.

Your proposed plan may save you a bit of effort now, but I really think it'll cost you later. And if this is a job that will be inspected by a code enforcement official, your current plan might very well result in a tear-out.

Good luck!

I agree with Mongo...Get a good tile person that is familiar with adjusting your drain system or, failing that, a plumber for that part. Have a mud bed installed and go with Kerdi or Hydroban for waterproofing. Properly built, it'll likely outlive you!