My Formula will maintain plane as slow as 17mph with tabs and drives down.
Your thread's title reads as if you are having trouble maintaining plane at slow speeds, but it sounds like you have no trouble planing slow; you sound more concerned about the wake at slow speeds.
There is no way to reduce a boat's wake other than to get more boat out of the water. That takes more speed or less wake. Whatever it takes to get your boat LEVEL will reduce the wake about as far as you can expect. Any more is just wishful thinking.
If there is a 30mph speed limit, then as long as you are running less than that you only need to be concerned about possible DAMAGE your wake causes. You don't need to be concerned about pissing people off with your wake. There's nothing illegal about a big wake, as long as it doesn't CAUSE or significantly ATTRIBUTE to property damage and/or injury. I know that's a gray area, but a judge I know said that he cannot imagine a court ruling against a boatowner (making a large wake) for injuries cause by another boater recklessly impacting that large wake.