How many active characters do you have? The more you have, the faster you can refine dilithium ore which you can sell to turn into zen. I started off with just one back in 2012 when the game first went to FtP. Gradually over time with various expansions necessitating new characters I now have a baker's dozen and cycle through them as efficiently as I can to farm the dil, with the exchange rate being what it is it's necessary now if you don't want to shell out real money. Just be aware that some items are unlocked on an account basis and some on character. Not a problem if you have a single character but it can become one when you create more.
I do know how overwhelming it must be when I look at all the things a new character has to do to attain the same level of development as the others and is why I now dread having to do it. For the established toons I need to customise them further as they've already unlocked all they are going to.
Fleet credits are awarded by contributing resources to fleet projects, the level of development determining what missions are available. Of course if you're looking to get actual fleet ships you'll need something called
Fleet Ship Modules, most of the ships in the store are fleet-level anyway. Fleet ships cost only 1 FSM if you own the store version of that ship but significantly, the store version comes with its special console and mastery trait, the fleet one does not.
Under the PvE Task Force Operations tab you can select missions which offer a choice of marks such as fleet marks, space patrols are good for those as they give dil ore, marks (selectable upon completion), and loot. Space loot is worth ten times as much EC as ground loot and is best offloaded at vendors as they give better EC prices than recycling things yourself. It can also be turned into salvage which you need to re-engineer gear to get better modifiers. As you've found, the exchange is where the huge sums of EC change hands since the good stuff in the game is not initially acquired with ec but one of the other currencies and is turned into it when trading.
What weapons are you running on your ship? They all have unique procs and will determine what kind of build would be optimum. Really with new ships always coming in I spend more time collecting their masteries than taking time to enjoy them (since the mastery has to be collected individually for every character) which makes double-XP weekends all the more important.