@scathach
Oddly enough, my Android phone can act as a hotspot AND be connected to a different wireless network at the same time — I sometimes have to resort to such a weird setup to use this phone as a makeshift VPN box.
I think the network you connect to has to be 5 GHz and the tethered one is would be 2,4 GHz — so exactly what you want! It's certainly flaky, you better keep your laptop, your phone and the original access point close to each other, but it does work.
@scathach
My phone also has a good AOSP-based ROM — could be the reason too.
Not sure it's worth the hassle, but if your phone is rooted and you have shell access you might want to look into this: https://android.stackexchange.com/questions/201024/how-to-use-wi-fi-and-hotspot-at-the-same-time-on-android
I never had to do anything of the sort though — I just touch the hotspot icon in the QS panel and… surprisingly it works 🤷