So, I've been a practicing satanist for just about four years and I am finally financially able to easily afford official membership in the COS. I'm also very interested in active membership. My question is, if I send in my application for active membership along with my application for standard membership, will it be taken seriously or would I be better off to wait a few months?

Yes it will be taken seriously and it will be read with care and attention.

There is no problem sending it in at the same time, providing you yourself feel you are ready to do so.