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

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.

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.

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.

Keep in mind LED lighting is probably the single biggest drain on wireless keyboard battery life, not like it doesn't exist or you shouldn't use it, but it will eat your battery life

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 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.

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