ErgoMechKeyboards

I dont think your going to find a keyboard thats is off the shelf that fits all those requirements. Besides the Advatage 360, theres the Dygma Defy for 500 USD (with all those extras you want, 325 with no tenting and wired, and its not coming out anytime soon). You can go to Esty and search for a "dactyl manuform"

Thebigskree (https://skree.us/) ~350 USD, Is wireless, hotswap, but not RGB, but the pcbs seem to support it. Going to have to contact them. RGB sucks up the battery fast, maybe thats why the option isn't available.

Cyboards ( https://www.cyboard.digital/ ) 245 to 385 USD, depending #keys, has RGB, but not wireless.

Wyderbuilds (https://wylderbuilds.com/shop/p/dactyl-manuform-build) ~ 300 USD, RGB, hotswap, not wireless.

Theres not alot of readily available wireless ergo keyboards. Gonna have to ask yourself do you really need it? If you drop the wireless requirement, you'll have Way more options, or go DIY.

https://ergomech.store/ Solfe RGB

https://keeb.io/ - Iris and others

https://falba.tech/ - Wireless splits available, not so sure about RGB options, if any.

Hope it helps.

Leo from beekeeb offers pre-build split keyboards like this one: https://shop.beekeeb.com/product/pre-soldered-wireless-dao-choc-ble-keyboard/ and it's within your 350$ range.
It should check all your boxes except a dongle and rgb. Regarding the travel case: He even offers a soft travel pouch in the store.
RGB on a wireless split keyboard needs a pretty big battery to have more battery life than a day or too, so it's difficult to find both rgb and wireless in a pre built one without raising the costs. Also some LEDs draw constant power without even being on which also adds to this issue.

Check out Keebmaker.com. They’re US based.

Proper split ergo keyboards are pricey, and selection is very small: https://kinesis-ergo.com/keyboards/advantage360/#buy-now

I'm currently building my own Redox because I can't justify spending that much on a Kinesis: https://shop.beekeeb.com/product/redox-v1-pcb-kit/

Maybe you can get somebody to build you a Redox for less than $350? Check Etsy, ebay.

I think the 2.4ghz is going to be the most rare feature. I found essentially one Etsy seller (https://www.ebay.com/sch/i.html?sid=vooooodooooo&_pgn=1&isRefine=true&_trksid=p2349624.m3561.l49496) and then the dygma. I think since Bluetooth in integrated in to microcontrollers, this becomes the simpler solution.

ErgoDox wireless from SliceMK (https://www.slicemk.com/pages/ergodox-wireless) is my suggestion. My current daily driver is a ErgoDox Wireless Lite -- been using it for about 10 months now. The guy behind it (series of wireless ErgoDox variants) is very helpful with problems on his discord. He also has a webUI configurator that works reasonably well (which I don't use because I prefer to keep my configs in git).

I recommend you to try corne v3. I has a lot build options, cases, wire and wireless options, etc. Also you can buy pre-built one on etsy

Not currently available, and the budget is not yet known, but it checks all your other criteria (except the travel case): https://x-bows.com/products/x-bows-split-keyboard

Otherwise, you can start with another existing option and adapt it to add what is missing. For instance, if the keyboard is built around a pro micro and you want to make it wireless, you can replace the pro micro with a nice nano and find a way to place a battery and power switch.