Ah the old misunderstood and misrepresented 12 minute tune like the old R3s. This really is entirely dependent on whether the O2 sensor is wideband or narrow. Narrow ONLY works at idle. i.e no load. The old R3s were NB, maybe the new are WB?
If NB, yes, you can let it idle and a narrow band will adjust AT IDLE. Won't do jack for anything else especially under load where you need that extra fuel
Personally I'd be real careful modding the exhaust on any 2020 vehicle without a new map or fueling adjustment. New EU5 emission rules mesn she's lean, real lean. That backfiring on throttle close? Thats most likely leaning out causing it, that's not the sound of performance either, unless you have a wild cam overlap on a drag engine.
Old R3 was marginal with the AFR and you could sort of get away with bolting a pipe on and while lean and backfiring you could get by. I reserve judgement with new EU5 engines.
End of the day though if it makes you happy go for it.