CO2 Delivery - Regulators and Diffusers

I've got a 21L planted tank and within the last week got the CO2 system going finally. I've got a Milwaukee 957 regulator along with a Mr. Aqua disc diffuser. Within the last couple of days I've noticed I can't maintain a constant bubble rate as measured by the bubble chamber.

I was expecting to be able to set a constant bubble rate and then forget about it. The problem is the bubble rate decreases WRT time when the diffuser is attached. I have to increase the pressure using a black, course adjust knob to maintain a constant bubble rate.

I called Milwaukee support today and he took me through the process of drilling out the orifice on the needle valve to maybe see if there was some obstruction. He asked me to take the tube from the top of the bubble counter and blow into it. He then asked me if I could force air through the diffuser. It was nearly impossible for me to get any air out through the diffuser. According to Milwaukee, my diffuser is causing too much back pressure. Their system doesn't expect much resistance to CO2 flow. He recommended I replace the ceramic disc (tiny bubbles) with an airstone (larger bubbles).

This thing is really a single stage regulator, the low pressure regulator has been bypassed. Apparently the needle valve is of no use (advised to adjust to wide open position) either since he recommended I adjust my final bubble rate using the big black knob only.

The event that motivated this post was I heard a pop and then heard a rush of bubbles in the tank. The pressure need to maintain the bubble rate separated my diffuser from the end of the flexible tubing. The diffuser did not break.

Milwaukee is sending a higher flow needle valve. It seems no matter what equipment was downstream from the diffuser, it still would have separated from the tube. So, I think I need to look at why the diffuser is offering so much resistance. However, a regulator with one stage bypassed and a needle valve that offers no fine-tuning is a bad choice at any price.

Would a larger (area) diffuser solve the back pressure issue?

I'm thinking a larger diffuser along with replacing the needle valve would be the way to go. Would replacing the existing needle valve with a precision needle valve make sense? I'm thinking get around 2.0 bps with the needle valve fully open and then adjust down to 1.0 or whatever using the precision needle valve.