Frankly, I can't be bothered to build anything but the basic assault armies and lots of them.* For SP, that's more than good enough. First comes the attack fleets that sweep systems for enemies, then they bombard for a few seconds while the assault fleets swoop in and begin the ground assault. This also automatically leaves 12 assault armies behind on each planet, a perfect garrison force during wartime and occupation force post-conquest if necessary.
Assault armies are cheap to recruit, cheaper to maintain, don't rely on any scarce resource, quick to mass produce, and do the job perfectly well. Due to their easy availability they are also the ultimate disposable unit: I don't care whether they live or die, so long as they get the job done. The only time I could see more advanced armies being a real advantage in some scenarios would be in multiplayer, where even a competent player will occasionally fight wars against enemies that act in unexpected ways that require readjustment of strategy, something that never happens in singleplayer.
And as for attachments? I've built a grand of one attachment so far to see how the attachment system worked, at which point I discovered that the attachment UI was thoroughly idiotically implemented to guarantee maximum micromanagement for minimal gain.
I'll use attachments the moment I can click an "automatically add this attachment to every army produced on this planet and don't change this setting under any circumstances unless I order it" button and not before.
And as for the government type that gives bonuses to army damage and cost? Paradox must have been on crack when they thought that was a good idea given the current implementation of armies.
The whole army-system could be eliminated from the game with little loss to the play experience as far as I am concerned.
Either that, or give it a complete overhaul.
* Okay, slave armies if playing a slaver is a decent alternative because they share the same strengths as the basic assault army.