I just realized how I'd left this thread and thought I'd update it. We found a great frame builder in the end and have gone ahead and had my wife's bike modified. Well, it's more a case of a new bike with a modified/re-purposed frame. The wife is very happy with the result. Except now the bike lives in the house, can't possibly go in the shed!!!! The builder managed to keep the existing geometry. There was one minor tweak made in that the new fork was made with a custom crown which enabled better tyre clearance without altering the geometry. Enables a 35mm tyre where previously could only fit a 28 or perhaps 30mm but only just. Both the fork and the rear drop-outs are now thru-axle and the build included GRX-800 2 X componentry and hand-build wheels.
My bike is in getting worked on now. The frame modifications and new fork are done and it will soon go off for painting. Mine was a little more complicated apparently as the seat stays needed to be removed and modified to make room for the disc caliper...