A detailed review of the current state of the game and its problems from the perspective of a genre

[fringe wind]

The game's main problem at the moment is production itself. The constantly increasing cost of both buildings and troops—especially the inadequate cost of troops as the era progresses—completely undermines the very essence of a 4X strategy game of this format. Why does a city with 100+ units of production on turn 100 produce a troop for 5-15 turns 4 th era? Yes, you can buy it back with gold and influence, but this is stupid and hinders the development of industrial civilizations.
You don't feel like you're making progress; as technology advances, it actually debuffs you. Instead of a 30% troop production buff for the middle era, which is traditionally about war, we get a 60% debuff for a 10% stat increase for a squad of recruits. This is idiotic.
Your previous games didn't have this problem.
The second problem is protectorates—why do they only have 6 slots? Who benefits from the opportunity to get all kinds of late-game bonuses? And why does the computer burn villages, depriving itself of population, 9 times out of 10?
The third problem is diplomacy. Four of the five factions are very willing and easy to negotiate with, often for free, anyone but the Swarm. There's no point in fighting anyone, and the computer doesn't care who we're friends with, who's in our protectorate, what actions we take, what technologies and improvements we make.
In fact, the only guaranteed enemy faction is the Swarm, and it's extremely weak in the hands of a bot. In all my games, on the second-to-lowest difficulty, it placed last, with the smallest territories, a weak economy, and cities. Even if you put three or four Swarms together, they'll still devour each other. I'd add diplomatic missions, incidents, preliminary relations with each other and minor factions, notes, and demands—see how this is implemented in the game Zephon.

You have Militarists, Religious Fanatics, and Genocidal Feudal Lords, all of whom behave identically toward hippie communists and each other, allowing free passage and even migration of the population.
And finally, I'd like more units and more branched development paths with the ability to replace single-tier options. For example, the Corals' archer option, which costs dust, is completely useless compared to its counterpart, which costs strategic resources. And you're forced to build an army instead of building in the capital.
Midgame battles only provide a challenge against neutrals, who are good at spamming. The enemy empire doesn't provide any challenge other than sieges. The game's defense is so good that neutrals and empires can't do anything, and that's not even counting the human faction, whose defense is several orders of magnitude stronger.
And lastly, luxury resources—their activation is tied to the entire empire, and the microscopic bonuses are simply laughable. In the midgame, they require 120+ resources at a production rate of 1-4 per turn to give +10 food. Isn't that funny to you?
You implemented this mechanic perfectly in Endless Space 2.
There, you adore luxury resources, build your game around them, and play around your starting location. They're important and one of the key mechanics.
I would return luxury resources to the level of city bonuses. For small cities, the bonuses are optimal. I removed the cost increase and introduced Ascension, with the same ability to move population between cities as in a spaceport.

And I introduced complex, large empire-wide bonuses for using several types of luxury and strategic resources at once.
Also, the ability to abandon a city district (suburb) is needed, either by converting it into a separate city or downgrading it to a fortification. This is especially relevant for the archipelago start and the retreat of waters.
And finally, all scouts, not just humans, need to be given a base 4 movement points, otherwise they are useless at any start except the archipelago.
We need more artifact slots. At a minimum, we need a second trinket slot, at least as a late-game choice in the hero tree, and/or a Tier 5+ technology. We could implement a system like our competitors in Spellforce, with different four slots depending on the hero class—some might have two weapons, some two armor, some two amulets, some two boots/pets/mounts.
Noted oddity: Dust Knights allow migration of their population, and the player can harvest them for food? And the knight harvests and sacrifices them? Very odd from a world-level logic perspective.

P.S. Overall, the game works great as a concept; it's missing factions and the aforementioned changes. Thank you for your work.
P.S.S The combat balance is excellent, although I didn't immediately understand the point with the corals, and didn't take advantage of the defense benefits, with defense stacks up to 80 units, but I figured it out on the 2nd restart
P.S.S.S The only thing that needs to be improved in combat is to prevent the bot from attacking if the enemy is attacking it. You're playing a lot in favor of the player. Look at the AOB 4 competitor. If the player attacks, no one will rush headlong and kill themselves against their carefully constructed defenses. Move yourself, sacrifice units, think.
And I would add the ability to retaliate, at least with 25% strength. This classic feature is missing.
And a variant of the ranged support trait, where a ranged unit responds to attacks on frontline units with a volley of 25% strength. Limiting it to units only, without heroes. At level 1, 1 support unit, at level 2, 2-3.
This would fit perfectly into the game balance for all existing factions and neutrals.

[simple sinew]

While I agree with your main point here, take notice that the game's ai is still being developed and isn't the best at warfare right now. It's drastically improved and actually declares war now if you piss it off enough, but it's currently operating on the "ai gets tangible buffs on higher difficulties" system to improve it in combat. The swarm is more competent when it's not debuffed compared to the player, as your settings have it (you said you played on a difficulty below the median, which either debuffs the ai or buffs you. I've never played below hero and am not sure which it is in EL2).

[fringe wind]

I played at the maximum possible difficulty level.

[simple sinew]

You said second to lowest?

[fringe wind]

I use a translator, there may be inaccuracies in understanding.

[simple sinew]

Ahh, ok. mb!

[fringe wind]

I looked it up, it's called legend in English.

In general, the balance specifically on the tactical map is about perfect, I would only redo a couple of small factions. above, I described the small polishes that are asking for

[simple sinew]

I would recommend changing this section from
"Also, the ability to abandon a city district (suburb) is needed, either by converting it into a separate city or downgrading it to a fortification."
to
"Also, the ability to abandon a territory is needed, either by converting it into a separate city or downgrading it to a camp."
In English, the individual buildings are called districts and fortification is the health of districts that you have to wear down in order to breach when sieging. The auto-translators changes made it difficult to parse and it took me a second to understand; suggesting this for the sake of clarity!

P.S: Great suggestions! I love basically everything that you wrote in this thread. Keep up the good work!

[fringe wind]

Thanks for the feedback on my feedback

[simple sinew]

It's definitely a less than ideal option to completely lose the territory and have to claim it again, but I just realized you could probably use the raze district option to destroy a ministry and unattached it. (far right)