I have a Nuphy keyboard on order that supports a 2.4GHz USB dongle in addition to Bluetooth. It was one of the main reasons I went with that brand.

My cheap Epomaker TH80 was $80 when I bought it, and supports USB-C, up to 3 bluetooth devices, _and_ a 2.4Ghz dongle connection. There are other reasons it might not be for everyone, but it's got a 2.4Ghz dongle, and those are actually pretty common in even the lower end of the mechanical keyboard market these days.

This keyboard already exists for the gamer market, it's called the G915, it uses a dongle.

I have a CIY Tester68 which comes with a 2.4GHz receiver, and also features Bluetooth 5.0. The hot-swappable board (no switches and keycaps) cost under $15 and powered by 2 x AAA.