So, did Quinlan strangle his own wife?

I think he tells his partner at some point that the killer died eventually. I haven't really caught any indication that he did it.

Thing is we don't know, we only know he was capable
and he preferred to kill by strangulation

Nah, the unpunished killer of his wife is the reason he's become contemptuous of justice and now is only interested in being right to the point that he'll ?, but that requires spoilers.

That's the impression I got. Never explicitly said, but the psychology of the man...

I think no. I don't think there's any outright evidence to suggest that he did anyway. I didn't even find it that ambiguous personally.

To me so much of the weight of this movie hinged on whether there was ultimately some good in Quinlan and if he strangled his wife then that's all gone. It'd render the film utterly useless imo.

I think it is mentioned that the killer died in WW1, no indication it would've been Quinlan. It is indicated that this event is the reason why Qiunlan no longer lets anyone go, not even the ones whose guilt cannot be proven.