Now, seeing as fleets won't be comprised of different ships, which would have given them different attributes, I've decided to go back to getting buffs directly from stars. But these buffs would be fleet-wide, if a fleet is "patrolling" certain stars it'll get it's buffs in all stars it's connected to. These could be stacked too, for example; if the fleet patrolled several red stars (red represents an attack buff) it's attack would be increased many times, albeit it this would naturally mean lower defense and production and what-not.
Fleets draw their ships from the garrison of every star it's connected to, it can also transfer ships from it's fleet into a garrison. This will be automatically managed based on the demand of the fleet and the demand of it's patrolled stars. So when a fleet isn't doing any fighting, it'll more likely fill up garrisons to increase happiness in the star systems (call it security), and when there is fighting it'll take ships away and would naturally make the system unhappy (no one likes constant war).
Fleets can also only move so far from system to system. But this won't mean that you can only attack so far away... If you can make a route that's connected to only your systems, then it's all good. However, if the fleet is trying to attack something far away it may have to pass through several systems, if you don't own these systems then the fleet will suffer attrition for as long as ships have to take that route.
I've written way more than I intended to here, lol! Here's a picture of some basic routes, fleets can also capture systems now, it's coming along! Thank you for reading. :)