SUCCESS!!!
In case anyone is considering a radian in a late model subaru, the trick is to position the plastic boot under the isofix mounts as this is the squishyist part of the seat (the doorside one works best) then push the seat forward as you pull the seat belt tight to get as upright a install as possible. I've also attached the tether (it just doesn't seem right to not in my mind no matter what the manual says about it being ok!)
Happy to report I can now drive without my preg belly touching the steering wheel. In fact it only takes up a bit more room than the Brio I currently use, which surprised me.
The real test will be if DH can drive. (I'm not the tallest person in the world!