Hi!
On a huge galaxy
My first advice: don't play huge all-abundant galexies. In the mid-game you'll find out there's too much work and too little fun in such a large game to be enjoyable.
For your problem this is also the solution. Play only in games where every player gets only 10-20 planets. A large all-abundant with 9 AIs should be about there.
For managing more than 20 planets you need more cash, or less spending. In the game you can do both:
- don't buy anything, but the first few factories on your HW. Build everything else;
- colonize with more pop, so colonies start supplying cash sooner,
- keep that pop happy (at 100% approval), and they'll grow with double speed, giving you more money sooner,
- research Sensors and build more ships to explore anomalies for some more cash,
- get econ and morale bonuses / resources / techs. They will all give you more money,
- if you're very low on cash, don't build anything on new colonies. Wait until they grow ~1B pop, and then start with some econ buildings (Trade Center's the best early choice)
There's more. Just check forums for early game strategies.
BR, Iztok