I don't know if this would be helpfull to you but I thought I would illustrate whats possible by using my own example from my last 2 games I played, and won.

In my last 2 games I was trying to get the TEC civilian research achievement which means zero military and zero defense research. So my fleets consisted only of capital ships with a handfull of light cobalt frigates used as defense since I had no access to hangers of any kind.

Even with only capital ships and no research done to improve them I still had little trouble taking out most starbases. The reason being that most starbases are not fully upgraded in their offensive abilities. And if you take care to keep tabs on when and where your enemy is building starbases you can make sure to try and take them out before they are upgraded. An unupgraded Starbase only requires a few, like 3 or 4, capital ships to take out with zero casualties. A mid upgraded Starbase tends to require maybe 5-10 capital ships depending on what exactly the starbase has upgraded and whether their are other forces to contend with at the same time or not. Only fully upgraded starbases gave me any trouble and even then I could still go around them and move on to the next planet. I don't find it too often that the enemy has 2 fully upgraded starbases in a row inless you are playing a very long game.

However even with a fully upgraded starbase I was able to take it down when I had a chance to consolidate all my cap ships so that I could attack it with everything I had which was 16 cap ships and about 30-40 light cobalt frigates. And my losses ended up only being a single cap ship. oddly enough I didn't even lose any of the light frigates. My Cap ships were ranked between 4-5 mainly with a couple that were 6 and 7. Again this is with zero research in the military and defense trees.

So why do I not think the starbases are OP? Because of what they are built for. They are meant to be able to defend against small to medium fleets to protect your flanks. So basicly they are meant to replace a fleet, but are limited to defending a single planet. This seems quite reasonable to me. It might be argued that a star base inconjunction with an enemy fleet makes it overpowered. However if the enemy has the bulk of his fleet in a gravity well with a starbase then it isn't logical to be attacking that planet anyways. Instead you should be attacking where he dosn't have his army and only has a starbase defending.

In games where I wasn't limiting myself to zero military research I found a great way to take out starbases is simply using the anti structure units. using just 5 or 10 of those with nothing else seems to be enough to take out even the most powerful starbases so long as it isn't the Vasari one since it can move into firing range. In the case of the vasari Starbase I just place my capital ships inbetween the starbase and my anti structure units to soak up the damage while the anti structure units take it out real quick. Again, if the enemy has a significant fleet present then thats not a very smart place to be attacking anyways.

I hope my example from my games can be helpfull to you in figuring out how to counter them.

actually, I have a great solution for the OP.  see the two SoaSE icons?  one says 'entrenchment' and one doesn't...click the one that doesn't. 

Because clearly Entrenchment is not for you.  You're complaining about all the things that make starbases's great.

Ok guys.......this is over. everyone calm down.

more like gas...........i had mexican jumping beans last night..sorry.

To be fair I think the build rate of starbases needs to be lowered slightly (ball park 25%) and then increase the amount the build faster upgrade gives you. They build just a little too fast for my taste currently.

But that doesn't change the fact that starbase rushers are counterable.

I always have a fleet early. Always. Cause I live in fear of a starbase rusher and WILL NOT be caught with my pants down.