All told I would not go back to a Milwaulkee reg either. Never been happy with the consistency out of the 2 I've used over the years, and reading up on it, this seems commonplace with them.
I'm right now using my CO2 on my planted FW but will be discontinuing it there and going back to a calcium reactor on my reef. The CO2 isn't really doing anything useful in the FW tank and all things being equal I much prefer a calcium reactor to dosing.
So I'm sort of interested in this topic and will follow along in the hopes that you find something better and I can leech off knowledge.
That aquariumplants.com one listed by Tyler above is the one that comes up again and again as one of the most recommended and trusted CO2 regs. I'd be tempted to try one of those if cost isn't an issue.
I dunno though. I suppose one alternative is set the bubble rate higher than you need and just use the Apex to cycle the solenoid as needed to maintain the 6.7 setpoint..