The oak will shade the CM someday and probably reduce the flowering of it. it will be a long time, though. You obviously know what you are up against. Decisions, decisions.......10-15 years from now, you may have more to make. Good luck!!

I also planted a dynamite in a bed next to the house this year. right now it gets full sun from mid day until late evening. I expect it will do fine for the first few years. But my bur oak is growing fast and will eventually provide heavy shade from the west. I expect the dynamite crepe will adjust to this condition, and adapt to getting mid-day sun only. I have one other mature crepe on the southeast corner of the house, and it has been very easy to maintain as well.

My experience with crape myrtles, and I have and have had many, is that they grow taller than the plant tags say. I have a Centennial Spirit (nice red by the way) that the tag said 8-12 feet when I bought it 4 years ago, and now I'm hearing much taller on this one too. It's at 12' now and only 4 years old. Part of the issue is they come out with new varieties and don't wait the 10-15 years to measure ultimate height, so they "guess". Also, growing conditions can make a difference too. It's really maddening when we get chastised for planting crapes that get to tall for their space, and I want to chastise the growers/nurseries for putting inaccurate data on the tag.

My mom has a lot of crepe myrtle. She has one red one I think its Tonto. I read on the tag it suppose to be a dwarf. It's such a slow grower. Its been on the ground for like 2 years now and is about 8 feet tall. It was already about 7 feet tall when we first got it. The color is nice though. I also notice it blooms later than the other crepe myrtle. The other ones are blooming right now, and I have yet to see it on the red one.