Does anyone have any luck with a complete and SIMPLE plant food formula? I
used lots of different products in the past, but would like to settle on
something simple and inexpensive.

A couple of comments.
I would highly suggest those adding house plant fert's, eg terrestrial
fertilizers stop asap.

It's fine for healthy well packed tanks doign well.
But if you add too much which most of you will at some point, you will
get algae.

It has as one poster mentioned already, nothing to do with the PO4
levels.
It does have everything to do with the NH4/urea content in these
ferts!!!!

I've added everything to high excess except Urea, NH4. No algae.
It does not take much NH4 either, by the time you test for it, often
it's gone already.

This is what was wrong with many eariler substrate enrichment
routines, jobes sticks etc. Many of these folks got green water.
You keep using the house ferts, you will too eventually.

I use three things, KNO3, stump remover, KH2PO4(or fleet enema as an
alternative), and you can use CMS which for 35$ will make about 50
liters of traces.
Get some boric acid(ant killer) from a pharmacy etc and add about a
teaspoon to the dry mix for Boron and kills ants with the rest.

You can also mix the Floruish or use it on alternate dosing days and
not mess with Boron etc.

So that should make things pretty cheap for you.
Also, sell your plants! Trade at the LFS, Aquabid them etc.

You should make more $ selling weeds than you ever spend on ferts.

Ag turf grass places often sell 50lb sacks of KNO3, KH2PO4, and trace
mixes also, roughly 20-30$ a sack. And a lifetime supply for you and a
few of your friends. So a group buy often helps.

Regards,
Tom Barr