I'd highly recommend the iClever BK05S. It's the same size as a normal laptop keyboard, so your fingers don't need to feel cramped. It lays flat and stable, it types well, it pairs easily, it charges with USB-C, and you can use it as a wired USB-C keyboard in a pinch.

Only problem is where to get it. Unavailable everywhere, spoke to the manufacturer on alibaba and they told me its EOL:( And if it breaks they probably won't repair it anyway.

It's a derivation of an old Kickstarter project: Jorno keyboard

I got the slightly smaller BK03 in Japan a few years back. I travel with it along with a mag-safe battery pack with a built-in kickstand, so that it forms a lovely typing setup even on a small cafe table.

An unexpected bonus is that the setup short-circuits a lot of reflexive app-switching and idle scrolling. With the phone propped up, I have to actually reach for the screen, so my thumbs don’t just wander from typing to home/app drawer gesture.