Actually, if one does a border crossing, even intending to travel beyond the Sonoran Free Zone, a passport is not needed. However, an FFM is required, and one's date of entry is entered into an immigration database. And honestly, no matter where I've been in Mexico I've not been asked to produce a passport, and only rarely my FFM. Thankfully crossing in both directions is, to date, and despite the political nonsense afoot, has been seamless. I too am a very frequent visitor to Mexico. Once a week at the very least and for one or two months every winter.
I cross into Mexico several times a month, and spend two months every winter there. I have yet to show my passport even once. I take it with me on long duration trips but that's it. On the one or two occasions I've been stopped (traffic check, military check point, crossing), all I ever show is my driver's license, FMM and proof of auto insurance.
I spend every winter there and just returned from a two month stay. I concur on the price of some food items. To be fair, dining in a restaurant can be commensurate with US, and even Thai prices. But I'm partial to small towns and street stall dining. I live just 5 miles from the border and do a fair bit of commerce cross border so I'm not strictly a seasonal visitor. I'm well attuned to the exchange rate and day to day pricing.
I just returned from a two month stay on the Sea of Cortez. I never felt unwelcome in the town where I visit yearly. No issues at all crossing the border in either direction. As always one's demeanor, dress and language capability can make a difference.
The 180 day entry is not a "visa". It is, technically, an FM (Forma Migratoria Múltiple) which is essentially an entry permit. For stays longer that 180 one would apply for a Temporary Residency Visa. That visa has some rather high savings/income requirements but is valid for one year and requires no other "check-in". For what it's worth I have never been asked to show, at a traffic stop or at a border crossing, to show evidence of my FFM.