Welcome to the board, mc.
I have been considering your plight and can relate to the feeling of being stuck in between having to focus on all the details and having to relax simultaneously. I am still working on it, and expect to be doing so for quite some time.
I could give you many details on that subject but am afraid I will only lengthen your list of "things to do" and complicate matters further.
One key element I have found very instrumental for me, personally, is relaxing the forehead...release the brow.
But there are many details to learn in the pursuit of "relaxing" in Taijiquan...
The shoulders are important towards relaxation...many people tend to hold alot of tension in their shoulders.
Try to consciously relax...We sometimes carry alot of built up tension around with us that we are not even aware of.
I think you can only really renounce to "thinking" about the details(such as form sequence, straight knees, no over-extensions, etc), once you've integrated them into your form and they become second nature, then you will do them automatically, without "thought"...which will then allow you to "focus" on something else...it really is a process...
Hope I could be of assistance.