Does an adapter for my 12x142mm thru axle frame exist to use QR hubs?
To convert your frame to QR you would need to replace your dropouts, which is problematic as they are either bonded or welded in place on the frame. This is basically a no go.

A much easier solution however is to replace your thru axle itself with another thru axle that gives ends that are compatible with older trainers.  These exist and are reasonably affordable (well affordable when compared to major frame reconstruction or or a frame swap).

A couple examples are pictured below:





Some creative internet searches should bring you to where these are available for purchase.  As Stack Exchange does not make product recommendations I have avoided direct links to the products.