Good points. You’d have to think Paine doesn’t make it to the 2021-22 Ashes, so I suspect you’re right in that they will probably be working backwards from series/tournaments like that.
Carey seems an obvious heir apparent with the gloves, but it would be brave to pencil him in as replacement skipper as well, given that presumably he and Paine don’t play in the same Test side (with neither being good enough to justify a spot as a specialist batsman as things stand).
Maybe they run with Paine for almost as long as he’s willing/able, and then switch to Cummins if they see him as a long term option, or Smith as a relatively safe pair of hands if they’re unwilling to make a long term commitment on a captain at that point.
Smith is close to 31 and Clarke went through to around 35 (with back issues, and a strong but inferior record to Smith), so Smith could potentially have another decent stint even if it didn’t start near term.