Already commented on the main game page, but I'm curious if you tweaked this later. I was able to get a steam deck analog stick mapped to the mouse in Pico-8 and was strafing and twin-sticking in no time. Clearly not as easy for setups less-tweakable than the SD, but still fun.
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I don't have a steam deck, but I do have a steam controller, and yeah it's crazy configurable, so mapping it to mouse and ESDF is the best way to get analog turning (although I prefer using the touch pad/accelerometer).
I made this update when I bought a USB controller for my Android phone. It can't do mouse input but acts like a standard game controller. So this gives me twin stick support on a hand-held device (unfortunately I can't get a Steam Deck here in New Zealand).
Of course the ideal setup would be to map the left stick to the Pico-8 mouse input, then you'd get analogue turning on a regular game controller. Should be possible, but I couldn't figure out the JavaScript to make it work.