Incomplete Catalytic Converter Monitor on otherwise flawless old Tacoma

I have a 2000 Toyota Tacoma with 250k miles on it that runs like a champ. But, after some repairs, the catalytic converter monitor (or catalyst monitor) has never set. Every mechanic has told me initially to “do a drive cycle” on it (or even “Google ‘OBD-2 drive cycle'”), but also that 50 miles of driving should cover the requirements. Yet, I have driven for months and probably 10000 miles and yet the monitor still shows as incomplete.

How can I tell which part I need to replace to fix this issue?

• I have already replaced the O2 sensors and the thermostat
• A forum discussion has suggested the thermostat or the Throttle Position Sensor (TPS)
  • The TPS would manifest itself in other issues, no?
  • I don’t know if they mean thermostat or temperature sensor
    • My OBD-2 reader reports the temperature at 180º-200º F during normal driving conditions
• Would replacement of the catalytic converter even affect the monitor? I don’t want to spend $300+ for a red herring
  • The Toyota dealership told me that my catalytic converter doesn’t have the right voltage to pass or the right voltage to fail, so that I would have to replace my catalytic converter. They then quoted me an outrageous sum to do so. I still haven’t decided if I believe them in full.