Thinking again about it, the main problem seems to be ELO is misused/misunderstood.
It isn't intended to be an exact science rigorously reflecting a player's skill or rank, but rather to give a rough idea of a player's experience.
IMO, having a different ELO for every possible configuration (number of player AND variants combination) would allow a better matchmaking, and Kingdomino is a good example of how unrelevant ELO is between 2P and 4P, but...
this would probably require a LOT of work, for little benefits.
The matchmaking might be a bit better, for sure, but thinking to a good system AND implementing it looks like a quite huge task. (playing a game's "new" configuration could take in account your ELO with other configurations, when did you play last, etc... what a mess !)
I'd rather suggest to put more distance between the players and their ELO.
I wonder why the "prestige" rewards have been implemented on BGA, but the recent "issues" with arena mode made me realize they are even worse than what I thought.
Note : the concept could be cool, but I think "prestige" is a really inadapted term here. Maybe call it "Experience" ?
You don't only get "prestige" achievements for futile things like having played X games, or getting your 10th victory... (even if it's after 1000 games xD), but also get temporary
prestige if you manage to keep your ELO among the top X players.
With things like this, having players grinding ELO, or dropping their opponent only to avoid losing a game is a normal consequence.
Personally, I came on BGA to find opponents, and didnt care at all about things like ELO... but reaching the 3rd (and probably 2nd) place on Kingdomino made me change my behaviour... Why ? Because of this silly ranking, and attached "Nth place" rewards.
I started playing tournaments in order to find 2P games (there wasn't many, 6 monthes ago) and end-up having a competitive playstyle I didn't even intended to have.
In the end, getting a "high level" incited me to stop taking too much risks, and more or less made me stop playing... which is quite absurd.
Of course, its up to me to choose my behaviour, and I could do better, but without (disturbing) pop-ups telling me I reached the top 20 / top 10 / top5, etc... I wouldn't even have glanced at the "ranking".