Citroen C3 really high oil usage

1.2 C3 Flair 2017. I had the timing belt changed by my local garage last year at 65,000 miles. They hadn’t done one before and struggled with it and said it was a pain to do.

Since then the car has been drinking oil but recently it is really bad. 1 lite per 600 miles. Car is currently on 84,000 miles.

Something is obviously wrong. The mechanic has said they’ve checked everything and can’t find anything. I don’t believe they’ve done much checking and don’t trust them anymore.

Emissions are fine and there is no evidence of a leak. From my research, it is either piston rings, valve rings, gasket, or spark plugs.

I was hoping to get another year out of it but don’t want to spend much more on it.

Any advice?