My Sierra's check engine light is on - code P0420, catalytic converter?

I'm hoping someone here can give me some advice because I'm getting conflicting information from different mechanics and I don't know what to believe. My 2016 GMC Sierra 1500 with the 5.3L V8 threw a P0420 code last week, which indicates the catalytic converter on bank 1 is below efficiency threshold. The truck has about 110,000 km on it and it's been running fine otherwise, no noticeable loss of power or rough idle, just that annoying orange light on the dash that won't go away. I took it to one garage and they said I need to replace the entire catalytic converter at a cost of nearly 4000 AED, which seems absolutely insane. Then I took it to another place and they suggested it could just be a faulty oxygen sensor that's causing the code, or maybe even an exhaust leak upstream that's throwing off the readings. I'm really confused because both places seemed confident in their diagnosis but they're telling me completely different things. I don't want to spend thousands on a new cat if all I need is a sensor, but I also don't want to ignore it if the cat is genuinely failing because I know that can get even more expensive down the road if it clogs up completely. A friend of mine who owns a Yukon recommended inspected GMC repair Dubai and said there are some good independent shops in Al Quoz that actually use proper diagnostic equipment to test the oxygen sensors and cat efficiency before just recommending a replacement. Has anyone here dealt with a P0420 code on their GM truck and can tell me what the actual fix was and what it cost? I just want an honest diagnosis without getting ripped off.