I assume you have played "tall". I play passively which looks like "tall done badly". The only thing that bails me out of it is:
- I like to role play BUT I need to Meta-Game and build to my military capacity [or beyond] during Mid and End game.
- This means I go for as many +Starbase techs & perks as I can. When desperate I've even used ascension perks to add +Starbases.
- Spam as many starbases with anchorages as you can
- Take as many +naval cap techs & perks as is reasonable
- As UNITY allows take the Grand Fleet unity ambition [edict]
- If time allows build mega structures to help. Dyson Sphere, Mineral Decompressor, and Strategic Coordination Center all help with building and maintaining a large fleet
Even with all of the above though I don't know if there is anything that can be done to play "passively" AND also be able to beat a Fallen / Awakened Empire if they wake up early. In my current game it's 2356, I've got 88K in combined fleets and just put in the order to build one more complete battleship fleet to take me to 110K fleets. While this is enough for the Khan I don't see how I can both play passively AND build a big enough fleet AND support it by 2400 [ish] to take out Fallen and/or Awakened Empires.
I've got more than enough alloys to support a fleet large enough. The lack of energy to support such a fleet though is the problem
For your energy woes, selling food and minerals on the market can go a long way. In the end though, your economy works and produces enough alloys from the minerals you can harvest ... or it doesn't and you die.
Watch out with the megastructures, they will sink a lot of your alloys, they can be a trap! It may be a better idea to get gateways going first, especially if you are sprawling and have multiple enemies.
Fortresses give a pretty decent amount of fleet cap, sure they cost some motes but can push some planets to 40/80 pops while providing FTL inhibitors. Crucially, they also don't cut into your early/mid game alloys like anchorages do (still, the motes *could* be used for more alloy foundries, so yea...)
Life really gets good when you can afford to have a single trade hub because your gateway network takes care of the rest

Only anchorages, shipyards and the odd citadel from then on!