P0420 and P0430 codes are usually pretty accurate. The cat should “consume” remaining oxygen in the exhaust stream, meaning that the downstream o2 sensor should show rich in most conditions. If you’re getting oscillation on your downstream sensor it’s pretty likely your cat is damaged.
If there has been an issue on bank 2 for a long time this could be why you have damage to your bank 2 cat, but not on your bank 1. I would definitely start figuring out why bank 2 caused damage to the catalytic converter before replacing it.
This is probably not a vacuum leak (such as an intake gasket like you suggested) because you’re going leaner at higher RPM, if it was an intake gasket you would expect to see a fuel trim improvement at higher RPM when the engine is making less vacuum (Assuming we’re only dealing with one issue here).
It’s important whenever you are looking at fuel trims that you are looking at them over a range of RPMs and adding the STFT and LTFT numbers together to get total fuel trims.