Specialty programs are going to cost. Probably a lot. Basically, these days lenders look upon any borrower who is unusual as toxic.

Your best bet is undoubtedly to manage to fit in a standard FannieMae niche.

You haven't said how big your retirement account is, so maybe you can't do this, but lots of retired people do.

Set up an automatic monthly withdrawal from an IRA. FNMA will then consider that as if it were a paycheck, as long as the account has enough assets to keep it up for 3 years. After the loan closes, you can then stop the withdrawal plan. If you are old enough you can stop it immediately. If you are too young, IIRC the IRS says you have to continue it for 5 years.