You may find 250 miles a day a bit much if you’re planning on stopping and looking at stuff. Definitely book accom, especially on the NC 500 route.
Border and Dumfries and Galloway landscapes are beautiful, not as dramatic as the north west but less crowded - there’s a lovely bike route from Langholm over Eskdalemuir. East Lothian coast, North Berwick, Bass Rock.
East Neuk of Fife - beautiful fishing villages, harbours. Would recommend Crail, and Falkland inland. St Andrews, maybe the Angus Glens and Kirriemuir. You could visit the Cairngorms and Royal Deeside on the way up. The ride over the Lecht is great. Lots of castles along the Moray coast, and villages like Pennan
North of Inverness, the Black Isle is worth a visit - you could take the Nigg Ferry which is a fun experience!
Fish and chips at Helmsdale, and the glens inland from Brora and Helmsdale are beautiful and often deserted. We’ve seen more deer in this area than anywhere else in Scotland.
Along the North Coast Smoo cave and the beaches around Durness. West coast is beautiful all the way down, Lochinver, Skye (have lunch at the Oyster Shed) Poolewe Gardens.
Further south, the Rhins of Galloway (on a clear day you can see Northern Ireland, Isle of Man and Lake District mountains.