Hello Brandan,
Are you self employed as a plumber or are you working for a company as W2? If you are W2 its a great idea to continue working there to be pre approved for your next investment. Self employment income we would need at least 12 months of self employment. You dont need to be at the same job for two years but you need to have work history of two years with no major gap.
IF your current income is enough to prequalify you for a fourplex or duplex then I would stay at your current job as plumber. To use FHA for a fourplex you would need to pass the self sufficient test which the rents need to at least cover your full monthly mortgage with taxes and insurance or better. If you decide to look into a duplex you do not need to pass that self sufficient test. If selling solar is going to make you more income then go for it and save money while you reach those 12 months of self employment. Best way is to get pre approved on your current scenario and look at the market to see what you can prequalify for so you can decide if you need to make more income and save more money or if you can start now. Hope this helps.
@Albert Bui @Matthew Kwan