I'm going to assume that by "transam", you actually mean the TransAm or TransAmerica Trail, and not just any old generic cross-country route.
Starting on June 8 suggests that W-to-E is fine. In my opinion, the first half of May is the best time to start for E-to-W and the first half of June is the best time to start for W-to-E.
Your plan for distance and time seems just about right. Some do more, some less. Your plan is just about average.
I prefer, and suggest, a lighter sleeping bag. On those nights when it's cold, you can always wear more to bed and/or use a liner. But a 20-degree bag will be way, way too hot for most of the trip. So why carry the extra weight and bulk.
One approach is certainly to mail home your warmer stuff after you get to Pueblo. That's a very reasonable plan. But don't send everything home. Although it will generally be warm after that, you will have some chilly nights in the Appalachians. I have known several people who sent their sleeping bag home after weeks of hot weather where they couldn't imagine it would ever get cold again. Don't go that far. You'll need it again.
I carried, and used, knee warmers. They came in handy on a number of cold and/or rainy days, and they weigh little. You could substitute tights if you prefer, but there is little reason to take both. I took a pair of lightweight zip-off pants, and never zipped off the legs. They were light enough that even on hot days, I appreciated the fact that they kept the sun, wind and mosquitoes off my legs. I took arm warmers too, but these are probably not necessary since you will undoubtedly be taking long-sleeve shirt(s) and a rain jacket as well.
How much clothing you take is largely a personal preference. You obviously need to take enough to stay alive. After that, however, it's a tradeoff between comfort and weight. My own preference is to accept a bit of discomfort occasionally to keep the weight down.
Keep in mind that every year is different. Many people, like me, will tell you how the weather was on the year they did it. It may be very different for you.