Sunday mornings are fine for travelling usually, but we tend to avoid arriving on sites between Friday lunchtime and Monday lunchtime because they are often full of weekenders during busy times. Arriving on Monday means a lot more choice of pitch because most of the weekenders have gone home. Arrive on a Thursday and the weekenders haven't arrived yet.
We have sometimes found that a site which shows full for a Friday, Saturday or Sunday arrival is half empty by Monday afternoon and stays that way until the following Friday.
Toweasy - a good stop on the M5 is Gloucester Services. Great caravan spaces both North and South bound.
As for timings, not too sure for your whole trip. But we've done Cornwall from Junction 3 M42, picking up the M5 at Junction 4 (Bromsgrove area) and get to Exeter/A30 in about 2hrs 45mins to 3 hours. And then to Indian Queens in about 1:30hrs or so.
PS - We normally stop at Lifton Farm Shop just before Launceston, just a few hundred yards off the A30 and a big open car park to the side. They do a nice breakfast (it's not cheap though) So we normally aim to get there about 10:00am. Then it's an easy last part of the journey, having refuelled and rested ourselves.
When we do our annual hols Durham to Dorset or Cornwall I like to set off from home between 10 and 11 pm, much more relaxing travelling overnight. Other times we travel anytime depending on where we are going.