I've managed to get Sooloos to release all my zones. The problem was that in Sooloos two zones were Linked (I had no idea: this must have been a leftover from many months ago). This meant that Sooloos refused to release either/both of them, which I guess is fair enough. Now I am able to play from both systems to the Meridian audio endpoints. Good result!
Roon and Sooloos still behave differently, but this is a design question:
In Sooloos, all the zones on the network are listed as selectable in the zone pull-down, and the re-pairing happens automatically when you select a zone that previously was used by Roon.
In Roon, the un-pairing of a zone results in it being removed from the zone pull-done zones entirely. It then needs to be re-enabled in audio settings before it can be selected from the zone pull-down. Roon needs to allow Sooloos to use the endpoint without losing the pairing entirely.
(For those who are interested, I have posted similar
here in the Roon forum, asking Brian to comment on whether they can simplify their end.)