Error code P0420 and OEM catalytic converter problem

I'm using a 2018 Tundra SR5 5.7l Flex Fuel 4WD double cab.

The catalytic converter on the driver’s side was stolen, and I had it repaired at a shop about 9 months ago.
At that time, it was replaced with an aftermarket part made by MAGNAFLOW, part number 49935.

Everything was fine for a while, but about 3 months ago, the engine check light(ECL) started turning on and off intermittently, and the engine became hard to start on cold mornings.
I took it back to the repair shop, where they told me there was an issue with the fuel pump, which they replaced.

However, since last month, the ECL has been coming on again, and they informed me that the error code P0420 was showing up.
They then replaced the downstream O2 sensor, but after some time, the light came on again, so they also replaced the upstream O2 sensor.

Despite this, the light still occasionally comes on.
I was told that the only way to fundamentally fix the issue would be to replace the catalytic converter with a genuine Toyota part, but the cost would be $4000.

I would like to avoid such an expensive repair if possible, so I have three questions:

1. Is the current catalytic converter actually defective?

2. Could driving with the ECL on have caused some sort of engine control issue, leading to the failure of the fuel pump? (When the ECL is on, the cruise control becomes unavailable.)

3. If the ECL comes on, is it dangerous to continue using the vehicle by canceling the code with an OBD II scanner?

After repeated breakdowns and repairs without resolving the issue, I'm feeling exhausted.
I would be grateful for any advice from you.