With the planet limit you also need to work toward strategic systems. For example, the T4 shipyard is massive, and the power requirement is insane. You’ll want to select a system (or systems) that will hold a T4 shipyard, and build them accordingly.
Each system and planet should have a purpose. Grabbing a 24km planet just because it’s 24km doesn’t make much sense if the resources are poor, the other planets in system suck, and it’s far from home. However, if you are tight on credits such a planet might be worth while because the huge population could be 2-3x the income of smaller planets.
As @Lord_Emperor said, Synits are best when they are at max colonies, or as close as possible.
So, my general suggestion for syntis is to expand as quickly as possible, but identify if a planet/system is actually worth holding. If you are grabbing planets for the sake of grabbing planets keep a list, so you know what to abandon when you find better planets. This approach means faster growth, but it’s not without drawbacks. Every structure built will be torn down, and all population and resources will need to be moved. This results in a lot of logistical work. I have over 2 dozen freighters moving resources and population around at all times.
The final point I will make is that really, it all comes down to playstyle. I enjoy the logistics, and managing freighter fleets. This means that I spend less time in combat. Most of my combat ships are running escort duty instead of hunting pirates. (it seems that if you don’t kill the pirate fleet then it doesn’t respawn, and it’s a lot easier to avoid a fleet you can watch move at warp 1 than it is to avoid one that just spawned). For others, I’m sure they’d rather do the opposite. There’s also a number of other workable ways to play.