I would rather not turn all into core worlds. I do not have a fix ruleset to turn colonies into cores but here are some general ideas:
- Existing core worlds might have high multipliers -> a loss of let's say 30% by decay may be worth it
- Core worlds generate control. You want to spam free colony ships every five turns
- Core worlds generate pops for colony ships, constructors and transports. Fewer pops = faster growth -> more cores = more new pops
- People on core worlds are happier (if you want them to be). More core worlds make it easier to win elections