From a tax point of view there is no such thing as an individual AGI on a joint tax return. There is no formula or method for calculating individual AGI because there is no such thing. As far as taxes are concerned your AGI is the AGI on your tax return. I don't know how the FAFSA wants you to arrive at an individual AGI in your situation, and I don't know how being in a community property state would affect it.
I couldn't find any information regarding your situation on the FAFSA web site. Probably your best approach is to contact them by phone or web chat and ask what you should do. Go to the following link and scroll down just a bit to the chat and call options under "Contact us." The hours of operation are shown there.
Federal Student Aid Information Center
There's one other thing I want to point out to you, although I know it will not help to answer your question. You mentioned your 2019 tax return. But if you are filling out the FAFSA now, the current FAFSA is for the 2020-2021 school year, and it asks for information from your 2018 tax return, not 2019.
