A long time ago, but:
Cornwall and Devon were the toughest parts of the whole ride, but were lovely.
I followed the old CTC hostels route in parts which mostly worked well.
I didn’t have a Garmin at the time, consequently there was more main road and getting lost than I would’ve liked.
The A82 can be unpleasantly busy. Pick your time carefully or consider the A9 cycle path instead.
Credit card touring ftw. I like a few comforts at the end of a long day’s ride. Plus you don’t have to carry much luggage.
Think about how you will get back from JOG.