Are you on the 12.9 or 11pro?I like your two device idea. The MK has been killing my wrist. I have to assume it is the small track pad and what I consider an awkward angle going from the keyboard to the trackpad. To be fair maybe I have a pre-existing wrist issue from years of using a mouse and trackpad.
I actually started using an external Apple trackpad with the MK. Kind of weird I know. Messed around with a Logitech MX Master 3 mouse on the iPad. Even easier on my wrist, but too big for laptop desk use on the couch.
When the ARM laptops come out I’ll be supplementing my iPad Pro with a new laptop which I assume will have a bigger trackpad. Maybe even replacing it with a smaller iPad for pure table use.
I actually did have some wrist pain last week from the 11 + MK, so I think it is the small size and awkward bend you have to put your wrist in, and the way you curl your fingers too. I think laptops have a much bigger area to rest your wrist on and that helps to minimize discomfort.
I watched a youtube video that suggested how to use trackpads more ergonomically. Basically, use the middle finger for pointer control...switch hands every so often, use a gel pad under wrist so you can keep the wrist as flat to the forearm as possible. It's really helped and I've not really experienced the pain since being more mindful of ergonomics. I do agree a sculpted mouse like the MX Master is probably the best, but like you said its not convenient on the couch.