Uh, the Enterprise could tank a ridiculous amount, even early on in the show, and she was shown to be able to deal out quite a bit as well. They blew massive holes in the first Borg cube they ever came across, and it was only their ignorance and insistence on following a very limiting rules of engagement that kept them from just blasting that cube to dust. The Yamato was lost due to a computer virus, and the Odyssey would have made it, even without the benefit of shields (talk about tanking) had the Jem'Hadar not kamikazed into her main deflector dish. The Enterprise herself was only lost due to hax and plot-induced crew incompetence. Those ships were not just pleasure yachts. And as far as we can tell, the only thing that was added to the Venture were a couple more phaser strips to add to the twelve she already had. There was also very little difference between the Enterprise in the main timeline and the one in the alternate timeline where the Federation had been at war with the Klingons since the Enterprise-C disappeared and she was a dedicated warship as far as we could tell.Except they don't. The Early Galaxy class ships were freakin pleasure yachts that had weapons tacked on after the fact. It isn't until 3 of the original 6 Galaxy class ships get pasted that we see Starfleet getting serious with the design. The ever labeled Venture refits.
And to their credit the Galaxy class got turned into a fucking BEAST!!!!
But as a Military force overall? Starfleet leaves a lot to be desired.
As far as Starfleet being a military, it always was, even during the soft TNG times - that doesn't mean it was a good one. as the saying goes, "good times make weak men" and all that.