A couple of years ago, I rode from Wells-next-the-Sea to Willingham by Stow (just north of Lincoln) via Kings Lynn and Lincoln (among many other places!) The route was mainly back roads and the Water Railway into Lincoln, it is on Strava
here.
My route across Kings Lynn was via the NCN route from Sandringham and I left over the bridge to West Lynn.
The only bit of really main road (without an accompanying cycle path) I covered was the short section of the A17 into Sutton Bridge (over the eponymous bridge!), which was fine, even the major roundabout at the other side of the bridge.
The Water Railway into Lincoln was fine for me on 700x25mm tyres, even the "summer" route at Bardney, although it's a bit rough and in future I will be sticking to the "winter" route!
You're welcome to pick bits and pieces from it. If you need any more info, feel free to ask!