Greetings Green Fingers, and welcome to the aching back club.  I've been getting shrubs out of pots and into beds - wrestling them out of their pots after several years was somewhat challenging.  I've had no luck here with rhododendrons, so I just wonder if you have our problem, of soil just not acid enough for them.  Azaleas seem to do better, so you might give one of those a go if your rhododendron does fail.
Mxx