I mean, unless you limit Thanos portrayal to his fight with Odin and surfer doing nothing to him as the be all end-all, and you reject the Guardians of the Galaxy and the avengers as well as the ultimates (let's leave it at that for now), there is no reason to think Thanos is simply out of top-tier threat range. Specially good showings top-tier range (Thor and Superman have scary good showings after all). 

But more importantly, you are missing the essence of what I'm saying. I couldn't care less if A was canon and not B. What bothers me is how you (as well as a lot of other posters) seemingly cherry pick which retcon excuse count and which does not. A clone or an avatar? is there really a difference in the plot driven here? Why accept one and refuse the other?

If, like you say, the only retcon you refuse is the action 586, I don't see the problem. Desaad was later said and alluded to have impersonated Darkseid several times as well as using Avatars.

But even if we take Action comic at face value and stop trying to excuse it with avatars and impostors, I do not see what the problem is. Darkseid's downfall was only possible via him eating his own omega-beams. A plot device I hate and that was used WAY too much, but at least Byrne was the first at the time. 

Back then, the omega beams were a super scary powerful attacks. Superman was fleeing it always and Darkseid was shown casually eradicating even  someone like Aggog who could treat Orion like a bug.

More importantly, since then we had final crisis, new 52 and rebirth... Are we really going to ignore all the occasions they clearly took to try different approaches to all characters each times?

As he is, Darkseid is now portrayed as a huge fight for a team like the JLA. Thanos is currently similarly portrayed when facing teams over at Marvel.