I know adj can come between more and than but I don’t know why the clause can replace in adj position.
The basic statement is
I will know more ... than I know now.
after I study is adjunct info and yes, it can come between more and than.
The phrase can also begin the sentence or end it:
After I study I will know more than I know now.
I will know more than I know now after I study.
You can also do that when more modifies a noun:
They watched more TV in one week than he watched in a year.
In one week they watched more TV than he watched in a year.
This is awkward and a little confusing but not ungrammatical:
They watched more TV than he watched in a year, in one week.