Hi Tom and welcome to the forums,
Your itinerary is a good one and quite popular. People who have done that, or the reverse, have had good things to say about it and I think you would enjoy that trip.
Ok, now my honest opinion. Its good, but not your best option. Your plan has 6 nights, but you say you have 7 to 8, right? So you could add a couple?Here's a few things to chew on:
1. Whistler is a wonderful ski resort and a great mountain getaway from Vancouver any time of year. However, it isn't a patch on the Rockies for mountain scenery and the usual advice from people here is that when time is short and you are already doing the Rockies, you can probably give Whistler a miss. If you really need to see it, you can do it as a comfortable day trip from Vancouver, or just have lunch there on your way down from Kamloops.
2. While highway 5 from Jasper through Valemount and Clearwater from Kamloops is good, the gold standard (IMHO) is highway 1 through Golden and Revelstoke. Given the chance of poor weather in May, it would very much be worth your while to drive the Icefields Parkway twice in order to improve your chances of seeing that breathtaking scenery in clear weather. And if you are lucky with the weather, you won't regret seeing it twice. This would mean you would head south from Jasper towards Field and then Golden.
3. I really liked Revelstoke and think its worth two nights in order to have a full day to appreciate the area.
4. Get informed about the National Parks Pass options. If you decide to go with the annual pass, then you would be best off to include as many national sites as you can, so that would include places like Glacier and Mt. Revelstoke National Parks, and the national historic sites around Vancouver (Fort Langley, Gulf of Georgia Cannery, Britannia Mines) in order to get the most from your pass.
So, like I said, yours is a good plan and you would enjoy that route, but I wouldn't put that forward as my number one plan.