Is there a way to create a variant (A/B) using a different template from the main variant? Or by uploading a saved UB page to be the alternate variant?
Bit hacky - but should work fine.
So you have your page.
Create a new page from scratch - i.e select new page, pick your template etc.
Then save and go to overview - on the options bit for this variant you can select ‘move to another page’ - select this and choose to move it to your original page.
It will show up there as a new variant - with your separate template