I love this route!
I'd suggest you add the A507 between Buntingford and Baldock. It has some quite quick bits, but also tight curves and slightly challenging visibility. Keep your wits about you, read the bend right and it's a joy!
You might also consider another, more minor road; continue north on the A507 from the lights in Baldock, and take the Newnham Road on the right just before the A1M services at Junction 10. Proceed via Newnham, Ashwell, Guilden Morden and Wendy to the A1198, where you turn right back towards Royston.
This is nice route if you are in the area. The A10 is the least interesting of the run but is added to make the loop but can of course be missed. Parts of the A1038/1039 are really nice with some great twists and elevated sections with pretty good scenery too. There are also lots of really pleasant little villages along the route and just off of it.