What is the easiest visa route to convert to a retirement visa after a short overstay in Thailand?

Get the retirement visa before you make your way into Thailand as that gives you 90 days of stay and relaxed time to sort out in 30 days the deposit for a 1 year Extension of Stay, rather than first running around to get a visa.

I think you might be mis-remembering what that actually meant.

You can't apply for an in country 90 day Non-Immigrant type visa while ON overstay.

You can have an overstay stamp from a previous entry to thailand as long as your current entry you're NOT overstaying

So come in Nov,

get the 800K baht transferred in to a thai bank account in your name,

go apply for a 90 day Non-O visa based on being over 50 (retirement) for 2000baht,

wait the under review period

go back on that date, get the Non-O and new 90 day stamp inked in to your passport

then when you have 30 days or less left on that 90 day stamp apply for the year extension for 1900baht