You know, if there was ANY power I'd expect to have researched multiple doctrine trees and be able to use whichever one they thought was appropriate, it'd be the Klingons.
Think about it. Doctrine researchers tend to be military theorists or experienced generals; the Klingons have a warrior ethos that produces
at the high end quite a few thoughtful warrior-poet types. Think of General Chang; I bet he was a doctrine 'research team' for the Klingons up until his death. And I doubt he was the only one.
(Probably an advocate of Base Strike, going by his idea of an 'ideal' campaign against the Federation from the Klingon Academy game)
Moreover, doctrine research affords the possibility of victory not through having superior technology or greater numbers, but by just straight-up being better warriors and tacticians than your enemy. That has to appeal to the Klingon mindset.
So I would bet on the Klingons researching doctrine as avidly as we research, oh, computers or xenopsychology. Their answer to "which doctrine tree do they favor" may well be "whichever one we want this year, and we can switch if we have to, given a bit of time to work on it."
The situation where there are fleets flying about to decisively battle isn't one that occurs without pressure or battles on bases (or shipping) first.
Yes, but a large fleet can cause massive trouble simply by being present in an enemy's space. If you fly out and park your fleet right between two of their major colony worlds, they have to either reinforce
both worlds with ships, attack your fleet in deep space, or accept a risk that your fleet will hit whichever world they neglected to reinforce.
They can't "sit behind their defenses and laugh" without either heavily reinforcing one place (to withstand the Big Fleet) and accepting the loss of many other places, OR without parceling their reinforcements out widely to many places, and risking defeat in detail. It's not as though a big Decisive Battle fleet is
unable to batter down starbases and the like. It's just less specialized for the role than an equally sized Base Strike fleet.
So I think you're being far too fast to dismiss Decisive Battle as ineffective or impossible. If anything it's likely to work better in Star Trek than in real life naval warfare, because your fleet can physically fly right into the heart of enemy territory if they're not stopped. On Earth with ships this is impossible, because even for an island power, the ocean is by definition the
edge of someone's territory, not the center.
The Germans could safely adopt Fleet in Being because there was no way for the British to sail their fleet to a position directly between Berlin and the Western Front and dare the Germans to do something about it.
How do you get a fleet to come out of its fortifications so that you can fight it? The only way is to threaten something that doesn't have a big fleet protecting it, or that it cannot ignore. In other words, before you can decisively battle a fleet you have to hit some fixed defenses. As Ixaria demonstrated, the best way to do this and get a nice decisive battle out of it is to have two fleets. But doing so against an enemy that you don't outnumber 3:1 is to invite defeat in detail. So you hit where they're not as best you can and try to luck your way into fighting them either before or after you hit a base. Either way, you're going to be hitting a number of bases, and if your luck is bad then you'll have to fight at a starbase or outpost anyway.
For combat on the scale we'd expect between the great powers, the fixed defenses of most systems are going to be a speedbump in and of themselves. You yourself ran this analysis in the context of Lapycorias some months ago- the starbase isn't going to help much if the Cardassians amass Combat 80 or 100 worth of ships,
which they can.
Defenses on the scale we ran into at Ixaria and Gammon are a bit more of a problem, but those were the product of the Licori madly fortifying the bejeezus out of their star systems over the course of at least five years of total war against the Ked Peddah.
Alternately if you are on the defensive, why don't you want to fight at your fortifications, regardless of doctrine?
As outlined above, if you have three or four fortified targets within range of the same fleet, dividing your forces to reinforce each of the fortified targets is a bad idea and invites defeat in detail. Parking all your forces in one fortified place preserves the safety of the fleet itself, but means that the individual fortified targets can be snapped up in isolation.
Flying out to accept battle starts to seem tempting- IF the enemy isn't obviously stronger than you are.
I don't think it's accurate to view "but the defender has fortifications" as some kind of hard counter to the potential of a major fleet battle to be decisive
without specializing in counter-fortification tactics.
Now if there was significant attention in the doctrine slides to the types of operations necessary to force a decisive battle, or even fluff about them, then I would be more comfortable with it. But there isn't.
There is now!
More seriously, the kind of operations required to force decisive battle are mostly the province of fluff that would have to be written during a war, rather than being the province of the tech tree.