WiFi-to-WiFi hotspot is a good idea, I don't think all phones can do that though. I once used my rooted Android to spoof my Wifes MAC address to use her in-flight WiFi she paid for after she was not using it anymore.

If I understand correctly, if your WiFi chip comes with 2.4 and 5 GHz bands, it's "trivial" to connect to the network using the one band and provide the hotspot on the other one - it already has 2 antennas - at least the issue wouldn't be the hardware.

Lots of hardware supports being operated in access point mode and station mode at the same time -- on the same radio/antenna(s).

Even the venerable (and ancient) WRT54G could do this with appropriate firmware.

IIRC, I've even used this functionality with Windows ICS on my laptop many moons ago

It's not necessarily an efficient function, but efficiency isn't always the most important thing.

(These days, I often travel with a Mikrotik device that I've set up to be a hotspot abuser, so as to provide myself and whoever is with me a slice of Internet while having the immediate appearance of being a solitary device. It does this trick very well indeed.)

AFAIK this only works if your hotspot device functions as a NAT and not a bridge