Mark and I had a similar “challenge” a few years back trying to find a suitable route for a young lady with a disability. We thought we had it sussed but found that her inability to safely ride on the road scuppered most ideas. Like you, we started to consider flattish off-road routes (Rothiemurchus was one of them).
Some other places you might consider depending on route length:
Caledonian Canal – towpath all the way from FW to Gairlochy
NCN7 from Callander to Killin (or at least to the top of Glen Ogle)
Loch Katrine (some hills but it’s all tarmac and you can get the ferry back)
Tentsmuir Forest
Culbin Forest
Glen Strathfarrar
Strathconon
I wouldn’t count the road to Glenfinnan as “quiet”.