I grew up in Wells, live away now but I know the area pretty comprehensively. Funnily enough I only traveled back yesterday after spending the week there for carnival.
You have a perfect route right there. Start from Wells quay and ride along the bank down to the beach, there's NCN route 1 that runs along the back of the pinewoods - you can extend this further once you get to the next car park at Holkham then come back and follow route 1 up into Holkham Park - going past the Hall (there were lots of kids on off road recumbents when I came through yesterday - maybe they now hire them out).
The route goes up to the south boundary of the park and then east, through woods to Cuckoo lodge and down a farm track into the town again. Back at the quay - pancakes and ale aboard the Albatros. There are some fantastic little options that run off from this route that will be okay to ride, even though they won't be marked on the map. I could sort out a gpx and details of these if you want to send me an email.