I have to agree with you. Whilst on a good day the road transfer between the two airports can be sub 50 minutes, whether that be on a coach or other, you just never know what traffic it has in store for you from 1 minute to the next.

Atleast with the train, although it takes a few minutes longer than on that good day on the M25, you can pretty much atleast have a stable timed journey. Heathrow to Farringdon on the Elizabeth Line and then connect straight on to Thameslink all the way through to Gatwick, I don't find it that bad of a journey at all to be honest. If you do use this option, try and remember to sit at the very rear of the Elizabeth Line train. Although it does often get busier at Heathrow Central and Paddington stations on tbis journey, it makes it very easy for you on arrival into Farringdon. You can exit and get the lift straight up to the Southbound Thameslink platform.