I should consider a higher difficulty level.
Still, fortified starbases are pretty much invulnerable in the initial wars, when the AI is mostly on fighters. Put armor on them and they literally don't die.
That way you don't have to put any research into silly things like "lasers" or "missiles" (what are those?). Also, it makes management a lot easier. You don't have to move around several dozen ships/fleets at a time.
Also, if you spam to 50+ colonies, it doesn't matter if you lose one or two to a massive AI invasion fleet. The point is they have to sacrifice several massive transports just to take one destroyed planet. Economically it's worth it.
Maybe it's the difficulty level, but the point is that reaction fleets take up a lot of resources and research to build. Instead, by spending ALL your resources on economy and basic starbase fortification, you have virtually unconquerable planets and starbases for a very long time, while simultaneously booming your econ even more.
Consider: the AI loves building fighters. They also love going after your starbases. So a single econ starbase with maybe 10 constructor worth of improvements (5 econ, 5 defense and fortification) can take out several dozen AI fighters before being destroyed (if ever), which is worth it (since the whole time your starbase is also generating you production). You're basically sacrificing cheap constructors for expensive AI military units.