MoO3 did a few things right. Race design is awesome and way ahead of 2 and any game I played with both unique effects and different race groups offering 'chassis'. Want to play gas giant dwellers thats a basic chassis, fish, lizards, humanoids etc. Great stuff.
Then the scale is great. It feels like you are really competing for galactic dominance, with hundreds of colonies and ships. Things you can do in the galaxy are great too, terraforming planets (and your races have their own, different preferred types), making new ones.
Finally some good words to be said about galactic senate, division into senate and non senate races, the edicts that can be adopted.
Unfortunately it indeed is a mess of UI, micromanagement and taxes on three separate levels (planetary, system, empire wide) plus you can inject money investment into colonies... Fun stuff if you are autistic eco nerd maybe.
So yea, MoO2 plays better, is simpler and endearing. But I just darn wish MoO3 worked too. Such a shame...