Not sure it covers the 3rd party concerns, but freefilefillableforms has been around for a decade or longer. It's fillable PDFs of the IRS forms with all the in

Not sure it covers the 3rd party concerns, but freefilefillableforms has been around for a decade or longer. It's fillable PDFs of the IRS forms with all the instructions available, and basic calculation abilities. No restrictions on income, etc. but federal returns only. You get an email from them within 24 hours of whether your return was accepted or not. It's still obnoxious but for easy returns it seems better than buying turbotax or whatever people use instead.