Nearly all comments are completely missing the most important point in the OP question: "The small form factor is important so I can keep it with me like you would keep a physical paper notebook with you". When one says a physical paper notebook, it means something that is small and portable and you can put it in your pocket or a small bag, not a 13" laptop that weighs 2 kilos that you need a special and when you ope…
As someone who has bought a OneMix 2s specifically for coding, I'd recommend against it - there are difficulties using it for typing, much less programming. I use it in an emergency for SSH'ing onto a server to fix something, or debug a live issue, as it fits in an inside coat pocket and I can set up an encrypted linux partition on it (with some difficulty, but it is doable) but unless you have excellent eyes and tin…
But, I understood the OP question as not about being productive but being capable of doing some small work if needed when on the move, a the device is small enough to be with you everywhere you go and not consider it a brick load in your bag after 10 hours of carrying it with you, but still performant enough to be able to make a patches and shorter coding session on the move.