Yeah it's pretty cheap, a 30$ bag will last me a couple years. The soil I get is already pretty high microbial count like you say you also get, and it may not be needed to also add the mykos, but it's always helped my grows a lot so it's not something I'm gonna stop using any time soon that's for sure
I was told mykos works WITH the other microbes, it doesn't compete in the same way as some other types do where one will dominate the other. From what I can tell this is true.
The granular makes it so when you recycle your soil it just keep continuing to get better with time, like you say, a soil that breeds it. My ideas on this topic have really progressed over time from thinking it's best to buy fresh soil each cycle to thinking it's best to save soil and rebuild it and continue the micro herd