Automation and low maintenance is the key to keeping your sanity... My main domain is impossible to make into low maintenance without losing much profit, but all my other domains require about 30 seconds to 1 minute per day to maintain.
And yeah, knight stacking is so easy once you have a decent income. In fact getting new units then becomes the limiting factor of how fast you can shit out new knights. I have a decent number by now, 170 in my main domain and 60 in my secondary, 230 total. Pretty good knight stack considering we did kinda start late compared to some of the biggest guys.