There are many levels of cause and effect.

Central banks use open market operations. When they want interest rates to go down, they print money and buy debt on the open market until the rates reach their target. This new money is the cause... the new interest rates are the effect. The new money (aka "monetary base" or M0) is then multiplied 10x by private banks, due to fractional reserve banking. That M1 money is another effect. The controlled interest rate helps the private banks loan that money out.

Open market operations only change the "portfolio" makeup of the private sector on the margin. New new assets(new money) are added to the private sector. There is no money multiplier due to open market operations itself. Banks will lend as long as they can make a profit and since banks can create deposits out of thin air, what you are saying is inapplicable to modern monetary/banking systems. Banks can always create deposits to meet loan demand.

Except for reserve requirements https://en.wikipedia.org/wiki/Reserve_requirement#United_Sta...

And once the reserve requirement is maxed out, the only thing that can permit more private money creation is central bank money creation

Right. Banks can borrow reserves to meet requirements. It doesn't prevent them from meeting loan demand. Money creation is congruent to loan demand. Also, reserve requirements are accounted for in the next accounting period not in real time.