Hmm so reduce tradeships down to 1, and strike craft to 1 per squadron? I suppose you could also double up the states of all frigates to reduce frigates in game by half. Though with those settings micro would become more important.
Reducing Tradeships will give you your biggest change.
For example, currently every Trade Port spawns 5 trade ships.
Lets say its a longer game with 8 players. And conservatly each player has 10 trade ports scattered about.
So we have 10*5=50 and 50*8=400
Right there you have a mid to late game fleet worth of ships flying about that the game is tracking!
Reducing this down to 1 gives you this: 10*1=10 and 10*8=80. Its a big difference. Also this does affect how the income rates of trade ports since the formula factors in trade ports that reach their destinations. Lowering the amount of trade ships also makes blowing them up better for you by causing a bigger disruptions in the max income rates of trade ports. Making raiding worth your time.
Now for SC, I wouldnt lower them down to 1 per squad. You could but I wouldnt. Reducing them by 1 based on the race. Off the top of my head Advent have 9 per squad. So lowering that down to 8 would be ok. You could then redo the squads total stats to compensate for the loss SC. So in this example, the remaining 8 would get buffed by 1/9 in all stats.
You can take this reduction down as far as you want but the lower you go it really can mess with balance.
Now for some numbers: Lets take my example for Strikecraft. Lets say you make a bunch of carriers, and now you have 100 squads of SC.
100*9=900 and lets say your fighting in a grav well with some enemies and they brought some SC in significant numbers.
Now there are 1400 different units just in SC alone!!!!! Not to mention Caps, Frigates, Titans, Tradeships, Refinery ships, Constructors, Starbases, Defensive Structures.
Now if we were to reduce SC numbers by 1 you save roughly 200 units that the game doesnt have to track.
As you can see, just tweaking these 2 things can reduce the load of the game, significantly. Ive seen vast improvident personally but your mileage will vary. You can take this less is more logic further if you wish but, reducing and condensing past a point will change how balance works and how the game plays over all.
I suggest you start with Trade ships, and if nothing else do that. Its the simplest and has the least effect on gameplay combat wise. TBH, I doubt you even notice the tiny difference in trade income, if any at all.
EDIT: Due note even with changes like this...your gonna hit a wall. This will extend the time before you hit it. Sooner or later, the bigger your map the quicker your gonna hit that wall, just due to the nature of having more ships and more trade ports.
After all every game has its limits. The limits for this game are high, but we just take it tracking all those tiny ships for granted.