A great book on this subject is “Start Small Stay Small” by Rob Walling : https://sivers.org/book/StartSmallStaySmall
Rob was running ten simultaneous businesses by himself, (and he has a few kids), diligently managing and tracking his time to run each one as efficiently as possible, and tracking it to make it easy to sell the business to someone some day with a specific intruction manual and estimate of how much time it would take to run that business.