Do note the Raspberry Pi doesn't have built-in LTE. Heck, older boards didn't even have built-in wifi.It's looking like the Raspberry Pi or a USB LTE modem might be my only real options.
Thanks everyone for all the replies. They are very useful.
If you want to use the RPi as hotspot, you still need to connect it to either USB LTE modem or smartphone or cellular tablet.
Also, USB tethering to Linux is broken on iOS 14 so you'll need to tether via another method.