I don't care much for warfare, though I do enjoy fleet maneuvers and strategy, but these can be handled at the Galaxy Map level. Once a fleet engages in battle, the only player input is giving the retreat order, but again this is done on a fleet-wide basis.
However, when I first got into Stellaris, the thing that impressed me the most was the fully-rendered space battles. I don't
watch them these days, because they're all very similar, but it would diminish the 'Wow factor' and perceived scale of the simulation if this were removed.
In fact, if I were a developer, I would add to the game a multiplier setting for the cost of building and maintaining ships during map creation. This will reduce the number of ships in the galaxy and, as a result, increase productivity.
This already exists, it's called the Habitable Worlds slider and Guaranteed Habitable Worlds, but players turn these up to get larger economies and fleets, then complain that fleets/pops are lagging the game. The same would happen if you let them effectively choose how many ships they wanted.
I'm pretty sure they're not rendered (since the frame rate tanks when you go into a system that has an ongoing fight, then recovers when you go back to galaxy view), but I'm also sure hey're still modelled – the game tracks the position of each ship, missile, and starfighter, and does battle computer decisionmaking, and tracks weapon discharge, and...
So the way to make everyone happy is to only simulate ships that are being monitored by the player. Ships in other systems act as a massed fleet, and in combat only their total health and firepower are checked. Then, should the player observe that system, the wave-function collapses and fleet hit points are distributed randomly (or weighted random) across the individual ships. In other words, we have Schrödinger's Fleets.
So I modded my game to give every empire 8000 fleet command limit.
The game still runs dramatically slower then 3.14. I do not think the issue is actually fleets TBH. Like yes in battles they are an issue but I do not think that is what is making 4.0 so damn slow.
Nice. Could you perhaps disable migration (or migration treaties and Federations) next? Migration creates small pop groups.