2013 Cruze ecotech fault codes

[rschap1]

Sister in law has a 13 cruze. Asked me to scan and take a look.
Has had P0420 with check engine light on for a long time now, couple years.
Has ran fine like that.
I assumed either an O2 sensor or actually needs a new converter.
But now she says it has started "stuttering" , sputtering, or missing at idle.
Mostly warm at stop lights.
Found the P0420 along with others when scanned a couple days ago.
Also saw; P0171lean, P0324 knock control , P2135 throttle position sensor correlation.
Was able to clear those with only the P0420 returning.
I think car has about 115 thousand miles if I recall correctly.
Most common maintenance done.
I thought plugs, but about 10 thousand on those and one I pulled looked good.
Wondered if there were any thoughts or common issues with that ecotech to start with.
Or do I tell her actually time for converter?
Think she was quoted $1200.
Appreciate any help or advice.
TIA

[rschap1]

Have to check with the wife to see if she has talked to her sister since I cleared the extra codes.
Wondering if still missing or not...