I'm driving to Scotland today. Normally I would go A50 west to join the M6 north, but last time I did this there was a 50 limit for about 30 miles that caused no end of delays.

Therefore I'm thinking of going M1/A1 north to A66 west.

Anyone got any advice on the best route?

Do the route on google maps, has red lines for any congestion

Not more red lines!!!!!

Sounds like you were caught in average speed camera hell. Looks like they're not there anymore:
https://www.trafficdelays.co.uk/a50-traffic-delays/

Which bit of chillyjockland?

For example, the route to Dumfries and the route to Kelso would be very different.

Take the train as the views are great ad you can have a wee dram.

Arbroath. Stuck to the usual route up the M6 via A50 and it was pretty clear. Couple of 50 limit bits but only a couple of miles at a time.

I would have gone up the A1 myself pas Edinburgh. Arbroath is as East as East gets.

Just looked on google maps. Same timescale but going East is 20 miles less. Not worth worrying about.

The east coast route is quicker/shorter on paper, but a lot of it is only 2 or even 1 lane, so tractors and lorries etc get in the way. The bastads.

Arbroath Smokies (haddock) are excellent from the pub by the harbour.
Saw a match there couple of years ago. The ground is famous for being the closest to the sea of any in Europe.