Depending on your carrier, you could do this without swapping SIMs -- as far as I know, all of the Big Three in the US (Verizon, T-Mobile, AT&T) all have "number-sharing" programs normally reserved for things like smartwatches. In fact, Palm specifically advertises that they work with Verizon and T-Mobile, and proposes alternatives for other networks (https://palm.com/pages/companion). If you're serious about this, it sounds worth looking into!
> Music, sure - it seems like the right size for a good MP3 player (remember that term, kids?)
I own a sansa clip plus. It has a 32GB microSD card in it. An out-of-the-box storage of about 20GB would make the Palm phone a downgrade from my MP3 player.
It makes specific points to say that it is meant for those who don't want the phone to be a distraction. I don't recall the camera specs, but I can't imagine it's good by 2020 standards. Music, sure - it seems like the right size for a good MP3 player (remember that term, kids?)
I like the idea of a palm phone. Part of me thinks about getting a phone like it, and then a Oneplus 7T phablet, and swapping the SIM like you would swap a watch for different occasions.
Actually, depending on the response time and the reliability of getting SMS and calls post-swap, that sounds tempting...