Colonies are easier to take due to fewer defending citizens and no defensive buildings, so this means that invasions would be more go big or go home with less probing attacks and outmaneouvring.
If you wanted to focus profitable conquest goals on Core Worlds without completely removing the lower hanging fruits, you could set it up so that whenever a Colony changes which Core World it is attached to, the Colony takes a huge hit to output that slowly reverts back to normal. Ways to remove this output hit, or speed up its reversion to normality would include ensuring it switches to a newly established Core World (when a new Core World is established, it could wipe all the output hits to its new colonies immediately), and restoring it to its old Core World (whether that be because the Core World has been retaken, or because the colony has been taken too), as well as the possibility of policies, ship modules, planet upgrades or executive orders. This would put much more focus on the Core Worlds without removing the colonies as objects for conquest, which would be odd in terms of immersion and possibly result in some weird strategies involving buffer zones of invulnerable colonies combined with hive mind, engineers and other decay reduction strategies.
Thank you so much for more tool-tips and the great way you're implementing them! I second the combat mechanics clarity point, and also ask that citizen expectations, why exactly they're satisfied or not, and what you could do to meet more expectations be added to the tooltips
