I’d probably go with a laptop too and just use the phone as a modem for it — I am typing this at a restaurant on a Linux laptop — but if you have a backpack, which you probably do if you’re carrying a non-foldable keyboard — you can throw a USB-C monitor with a folding cover in it. This sort of thing:

https://www.amazon.com/MNN-Portable-15-6inch-Ultra-Slim-Speakers/dp/B0B9NNWXVP

I have a 200 Wh power bank (without an inverter, to help keep the size and weight down down). Similar to this:

https://www.amazon.com/Anker-Portable-Generator-Traveling-Emergencies/dp/B0D62P85ZR

That won’t work if you’re on an airplane (which it looks like is the case from the image), though you are allowed to carry up to two 100 Wh power banks under FAA regs.

If you use those, you should be able to set up a hub — this sort of thing — that permits pass-through charging, power everything off the hub and use wired gear, and thinking about individual charge on a bunch of disconnected battery-powered devices goes away. I don’t do that myself, just use a laptop as a hub, but for a phone-as-a-laptop configuration, that might make a lot of sense.

Another benefit of going wired is eliminating the Bluetooth privacy concerns — Google or Apple aren’t plugging your location into a database just because someone with an Android or iOS phone using location services happens to be in your general vicinity.

Though if you’re on an airplane, they may provide power at the seat, and you might not even need the power bank.

EDIT: The phone does need to support USB-C DisplayPort Alt-Mode if it’s going to use an external monitor. searches Here’s a list of models.

EDIT2: Important caveat, as this is !linuxphones@lemmy.ca — external displays using Alt Mode work on iOS and Android. I haven’t tried to look up what the state of affairs is for GNU/Linux, don’t know what the hardware support is like there.