OK. So we have Emperor Konstantin, who was pretty garbage at ruling Poland,
Which of the Russian viceroys/governors of Poland could boast some noticeable success unless by “success” you mean crushing any notion of independence? Constantine was following Alexander’s policy and
personally was rather fond of the Poles (to a degree that he forbade the Russian troops to act when uprising started and openly cheered the first victories of “his” Polish troops) but this did not really matter because the whole union thing had been fundamentally flawed on
both sides and could last only as long as the Russian side was leaving Poland completely alone with its Russophobia, an army commanded by the former Napoleonic generals and the government confusing the Congress Poland with France.
This was OK by Alexander (IIRC, he already had some problems with the Polish interpretation of the constitution which he wrote) because during the last years of his reign he pretty much gave up on the governing. But this could not last forever and any attempt to make Poland a meaningful part of the Russian Empire was going to produce an explosive reaction.
and also hated court etiquette.
Really? He was following it in his communications with AI and NI. He was just of opinion that the etiquette and common good manners does not apply to his communications with the subordinates and those socially inferior. This on its own would not be a big deal (the whole XVIII century was full of the similar examples) if not his fits of the erratic behavior: one of the main reasons for the plot against Paul was his instability.
He also abstained from politics which he can't do as emperor...
Politics was Alexander’s exclusive domain and Constantine was not invited into it. However, within the allowed limits, he was rather vocally preaching peace with Napoleon, before, in 1812, he became even more vocal his opponent. So annoyingly and offensively vocal that Barclay kicked him out of the fighting army.
So there is no valid reason that Constantine was going to abstain completely from the foreign politics but, OTOH, Alexander’s excessive involvement into it was a complete disaster for Russia: hundreds thousands dead, huge financial expenses, moronic idea of the Holly Alliance, subservience to the British interests (as soon as the dust settled he got a big F—You politically, economically and even in PR) and, cherry on the cake, Congress Poland (two wars and countless problems domestic and international). The same goes for NI who was also actively involved in the foreign politics and ended up with the CW. So, if Constantine manages to find an intelligent Chancellor who is not obsessed with the “European balance” and unprofitable alliances, lesser activities in the foreign policy could be good for Russia.
As far as I can tell, the main argument against Constantine as an emperor is “an idiot on the Russian throne”, which is a false premise. The problem would be (and was) “an
active idiot on the Russian throne”. The
active ones proved to be quite costly both in the short and long run but a ruler who is doing as little as possible is causing a minimal damage (*). Constantine limiting his activities to inventing the fancy uniforms and being rude to the Guards (to be fair, apologizing after the outbursts, which was more than any ruler before or after him did) would make a reasonably good monarch. 😂
(*) A though expressed by a famous liberal writer of the XIX who also happened to make an impressive bureaucratic career and knew tye subject from all perspectives.