Just from a Cub perspective, I can't find a single need that would fit. The goal of moving Bellinger is to help the team get better and I'm fairly certainly this would make the Cubs worse (not shooting the messenger, just the message - this type of a swap has been bandied about in Yankees media recently).
Stroman was bad last year. Almost everything dropped for him, and many to quite concerning levels. His chase, K%, whiff, and xData are all really terrible. The goal of the Cubs pitching is to add someone to the top of the rotation, and if you go the two-pitcher-route, ensure someone like Assad ends up in the swingman/6th guy role. Stroman is likely no better than Assad at this point and might be worse. And if he makes 140 innings being bad (which, if you're trading for him, you're likely using him) then you get him 2026 guaranteed! Maybe it's a mechanical flaw, but the Yankees develop arms well, so I'm not so inclined to think this is anything but natural decline.
Bellinger might be expensive, but is objectively still pretty good. He fits a general need for the Cubs and can play multiple positions. There's a much better chance Bellinger opts out in 2026 too. If you trade him, then you'd need an upgrade and unless the Cubs are pulling a super-secret smokescreen on Soto, none of the other FA OF'ers offer a clear upgrade. They might change the formula; Santander trades HR's for defense, but they're effectively the same kind of top-line value.
The Cubs aren't so desperate for money that a Stroman ($18m) and Santander ($18-22m AAV) is better value than just signing Kikuchi ($15-$18m) and keeping Bellinger for a year. They have $50m, and could still use their major prospect depth to trade for another pitcher.