different bike for kickr?

To answer the question you have actually posed, yes it's something I would think about. My understanding is that in simple terms, as a chain stretches, it wears the casette faster. So if you've done 000's of miles on the turbo, then put on a wheel with a brand new cassette, you're probably going to wear that cassette very quickly because the chain will be stretched. The solution would simply be to keep on top of wear and replace chains (and cassettes) as soon as they need it, and especially before switching to a new cassette.

I've built many bikes for customers from frames to use on smart trainers and always the customer have matched the frame geometry as close as possible to their Road use bike, then we've set it up with the identical specs.... usually when they emerge from hibernation they are fine on the road if a little stiff and blowing out of their rear ends a bit quicker than they expected by being back out there

As long as the cassette on the trainer and on the bike are at about the same stage of wear, your gears are well set up and everything is kept well lubed it shouldn't be too much of a problem. A budget new shorter stem for your trainer bike would be cheaper than a new cassette though, worth trying to see if you can get the geometry right? Also although the power numbers are the same road riding is a lot more wearing on the knees than trainer riding in terms of shock absorption and less smooth pedalling (slowing and restarting for traffic, junctions etc). I definitely found during lockdown that after a month of daily 30-50kms on the trainer I felt pretty stiff after the first rides back on the road.