Chevy Spark 2011, high LTFT at idle, MAP engine

Hello,

Chevrolet Spark LT GPLi 2011 ~50.000KM, speed density engine, high LTFT at idle (between 25 and 30% constant LTFT, STFT are between 5% and -5%, but sometimes at -15% at moments I couldn't explain (temperature?)) but seems good under load, I find the rpm low at idle (600rpm).
This car has a LTFT that settles at 5.47% as soon as you exceed idle engine speed (this seems to be a factory behavior), so I can't rule out being lean all the time. STFTs on cruise are well between -5% and 5% the vast majority of the time.

-> The problem occurs in both petrol and LPG mode, so I exclude fuel supply problems (the pump (which includes filter and regulator) has since been replaced for another problem, but that hasn't changed anything).

-> Sensors (temperatures, MAP, O2) return consistent values. The upstream sensor has been replaced, the one removed is still good and the sensor is white (so a priori true lean condition). I also tested with another IACV (Idle Air Control Valve) and another TPS (Throttle Position Sensor), no change (they were second-hand parts).

-> No intake leak highlighted by the smoke machine (and given the low RPM and that it's a speed density, I can rule it out, I think?).

-> I checked the EGR valve and the evap valve, and didn't see anything unusual.

-> I tried the smoke machine in the exhaust, apart from a very discreet hissing sound around the EGR, nothing to report. But in the meantime, the thermostat housing (which holds the valve) has been replaced, along with all the gaskets as it was leaking, as well as the thermostat and the ECT.

I'm thinking of looking at the condition of the spark plugs to see if it's an exhaust leak, in which case the plugs should be black due to fuel richness. I'll just have to be careful, as for a while I had a crank no-start problem (which came from the battery). It could have affected the spark plugs.
I also have a problem at cold start, where the engine fires straight away but can sputter for a few seconds before stabilizing properly. I don't think it has anything to do with that, though (maybe it's from fouled spark plugs).

Thanks!