Fixing my disorganized employers Quickbooks, HELP

I took over my employers QuickBooks in Oct, 2020. I am STILL trying to get things under control, and have not been successful. My employer set it up in 2018, and never once has done any reconciling, Year end tasks, Invoicing, bank transaction categorizing etc. His personal bank account is his business account, and I don't know how to set those transactions aside so they don't affect his Bank Balance VS his Quickbooks balance or his business spending. Right now its out by over 400K because of this. What can I do? Where do I start? I've been trying to just enter his invoices and at least his employee's data, but he has no desire to change the way he runs his business/ bank accounts. He wants it to somehow magically be fixed, and short of starting over completely I have no idea what to do. I have training in Sage accounting, but somehow Quickbooks is entirely beyond me, and I actually hate it but can't justify telling my employer to pay for more expensive software, when it will just end up as messed up as this one.