Hey. Just spotted this. @ThomasAR
So I do this stuff all the time. Swapping the MTB with its 12sp gearing on to (a Kickr or a Neo in my case) trainer. I don’t imagine the Core is any different.
When I’m not feeling like getting my hands dirty I just swap the bike on, if it’s my hardtail I have to take the brake caliper off (2mins). Tweak the gears if they need tweaked and off I go. 12 into 11 works fine for me … I’m not using the whole cassette anyway. And actually if I’m using ERG then I just use the best chain line and leave the gears alone. For non ERG I use Levels0-9 and make sure I have half a dozen if the gears working. It the difference is so small between 11 and 12 it just works fine.
When I can be bothered swapping the cassette I have one normal mount 12sp cassette that I keep for trainer use. It fits on the normal freehub - no need to change to XD. It cost me £70’a couple of years ago - was their lowest tech ‘NX’ level.
Benefit is it doesn’t wear out the nice SRAM cassettes that are expensive while Riding the trainer too.
They’ve been available for a couple of years now.
Then you have all gears and level mode to play with.
This is all do’able at little cost relatively speaking. Though looking at prices now cassette prices have gone up along with everything else. Maybe find a Sunrace one.
Pm me if you want any direct advice on installs or anything.