What you're saying is either total BS, or there's something you're not telling.
With a credit score of 800, and a verifiable income of $300 K, you can get a loan from most any bank you walk into. FNMA and GNMA don't need to be involved. Most any bank will make you a loan on their own hook.
So why can't you get a loan?
A. You want to borrow a million dollars on a home appraised at $250K.
B. You don't have proof - W2s, 1099s, 1040s, etc that your income is really $300K.
C. Your $300K income is one-time: Inheritance, Vegas winnings, settlement of a lawsuit.
But....if what you said and implied is actually true:
800 credit score
$300K verifiable, sustainable income
and if your present debt is >$100K
And if your present net worth is any + number
And you still can't get a mortgage loan
Then message me.
I'll lend you the money, up to 90% of appraised value, on any $500K house you like.