The game is really solid. It seems like the dev team has made a disciplined effort of making sure that the mechanics all work fluidly. It plays super well. Quality.
My first few play throughs were with an "easy" galaxy. After I posted the review, I played again on "bright" difficulty. I also selected a lower level of habitable worlds so that there would be more early competition for them. These settings made the first half more difficult as I had to colony rush and then expand without triggering a war. But once I had enough fleet logistics I was able to steamroll through again.
If the Galactic challenges were things like "survey x anomalies", "discover x technology", then yes I did those. I liked them also. A good mechanic. I think that it may have been more fun if the rewards were things that you couldn't get any other way. Also they should expire so there is a degree of urgency. I mean I'm going to survey the anomalies anyway as a matter of course.
BTW I love the mechanic where certain ships (transports, constructors, colony) take pops. Its adds some strategy and makes developing pops important.
My issue with the endgame was in a few parts. First, its super easy to swarm an enemy civ because the cheapest fighter can take over a regular colony world. So you can attack all of their colonies at once and then pick off their core worlds with a few transports. Second, the fleet mechanics (which I really enjoy) allow you to develop an overwhelming advantage over the enemy fleets if you are patient. Once you have that it becomes a tedious mop up job. It sort of feels like that japanese game Katamari. The more planets you roll up the easier it is to roll up planets. I think it would more fun if you had to digest new colonies some way. Conquer a civ but then you have to spend time using diplomacy to avoid war while you integrate and build up your new colonies. Third, I think there need to be new resources and tech that are locked away in further more dangerous sectors so that you are always tempted to overextend to get that new tech.
The game is so good already. The recruitment mechanics are great, especially early game when you have to decide between rushing a colony ship or buying a leader. I play civ VI also. But it always bogs down with micromanagement. GCIV felt so much cleaner in the endgame. I really appreciated that.
I think the biggest underdeveloped opportunity for fun is the little story lines. It is super fun to decide whether to let the crew uplift themselves. But nothing ever happened with that. No super human tried to take over a colony or depose my government or start a new religion that makes people unhappy. It would seem relatively easy to add such consequences in and maybe organize them so that they are more likely to occur if the player is too successful. Also I think the stories would be better if it wasn't spelled out so clearly what the immediate consequences were. Once you realize that there are no hidden future consequences coming down the road, it just become deciding whether you need credits, trade goods, etc.
It doesn't seem like the AI cheats--which is appreciated. By that I mean, for example, the AI doesn't seem to know automatically where the good planets are. I think that is great. But I think that the game might benefit from a story teller (Rimworld-like) approach to challenges. This might be an easy way to extend the little stories. If the player has too big a katamari ball of planets, let the uplifted crew throw some planets in to rebellion. Or if the player didn't uplift, have one of those crystal intelligences reach out...wait you don't have any uplifted pops smart enough to speak to the crystal -- too bad for that colony.
I hope that helps understand my experience. Please continue to do the good work. I'll buy your DLC when you put it out. : )