I am in this boat. iOS 26.1 and macOS 15.7.2.
I haven't had any issue with the mix-and-match. I can still use iPhone Mirroring to access my phone from my Mac. I guess one thing is that macOS 15 doesn't "know" about the text message classification system in iOS 26 (spam, transactions, etc.) so they all still get lumped together on macOS. That's a minor thing.
I went "early" to iOS 26 because I wanted to be able to use the call screening feature.
I'll upgrade the Mac when I am more satisfied that the transition to Tahoe will be smooth.