IIRC Goretex is advertised as a breathable fabric. Can’t say how much it really is as I’ve never worn any.
We, too, get lots of warm rain in East TN. I sometimes use a standard “Fish” slicker. Here is what my wife and I both use: http://www.cowboyshop.com/old-time-pommel-slicker-p-358.html No rain gear that works is really comfortable if it’s hot and humid but these do the job with reasonable comfort and value.
I also have a reproduction of a 1912 Marine Corps poncho. It’s a pretty heavy rubberized canvas and is warm on warm days. The company that made it is no longer in business. I don’t know of any other source.
For colder weather I have a reproduction of the M1908 Mounted Coat. http://onlinemilitaria.net/products/6688-US-M1908-Mounted-Pattern-Raincoat-for-Cavalry/ It is an excellent product but it is heavy. And it was designed to be worn over the soldier and his equipment and protect the cantle roll (with bedding, etc.). So it was designed by Fashions by Omar (the Tentmaker). It is HUGE. I’d recommend a poncho before I’d recommend this in a very warm climate but it is an alternative.
I’ve tried rainsuits before (separate trousers and jacket) and for ground work they are OK but are not suited to mounted work as they are not cut to allow for the position you must assume in the saddle. If you go a couple of sizes larger than you’d otherwise need they can work, but now you’ve got a bunch of extra material where you don’t really need it (or even want it). The legs also tend to be short due to the trousers “riding up” and that reduces protection of your legs and boots.
I’m sorry to hear of your problems with Muck Boots. I’ve always found them to be of good quality. Presently my wet weather boots (and we get 50" of rain, more or less, per year here in East TN) are from Noble Outfitters. https://www.nobleoutfitters.com/products/muds-cold-front-womens-high I put up the woman’s version as I think that’s what you will be looking for!!! Mine are now three years old and show virtually NO signs of wear and they get worn quite a bit. You can find cheaper options, but you do get what you pay for. 
I’m sure others can give you additional options.
Good luck in your search and I hope your rain abates soon.
G.