But the stream will consume the cellular connection, because you’re using the phone’s wifi for the hotspot (the phone only has one wifi interface so it cannot use it both to connect to the local LAN and for hotspot).
This is not necessarily true. It’s almost always possible to use the 2.4GHz band on one side and the 5GHz one on the other and multiple networks on one WiFi interface isn’t impossible either.
Modern Android devices can provide hotspot of the connected WiFi without any modifications. For some anecdata: My FP4 with LineageOS 20 and a Samsung A50 (Android 11) can both do it.