Then how were the Rebel fighters able to get so close they were scaping the hull? No way could a starfighter fly through an active shield.
Did you miss the giant honking superlaser dish connected directly to the reactor core - and you can bet ST sensors are good enough to pick that up on scans - that just screams bullseye? Also, ST ships use the same power source as their torpedoes, so a starship is basically a mondo torpedo in a pinch. Wouldn't even be the first time a starship got sacrificed to take out a planet-killer.
On the first: SW shields are two separate, mutually exclusive types- you cannot have both Ray and Particle shields active simultaneously. Ray shields- which protect a ship from energy weapons such as Turbolasers- can, therefore, be penetrated by fighter craft, as turning your shields off to reconfigure them in the middle of a battle is monumentally stupid and, moreover, would leave you completely exposed to weapon fire either from a supporting capital ship, or from those same snubfighters skimming your shield perimeter and blasting away at their leasure.
As for the second: given that such tactics were unable to destroy a single Borg Cube, barely a kilometer to a side, it beggars belief that doing the same would do much of anything to a ship whose diameter is measured in
hundreds of kilometers.
I mean, maybe if they chucked the spacedock at it, since those keep giant reserves of antimatter on hand, but otherwise it's the 'killing a Cube' problem; ST ships are just too small, and their weaponry too precise and contained in damage, to meaningfully oppose a vessel the size of the Death Star.
Tarkin would almost
certainly move the battle-station into Earth orbit, and compel compliance under threat of turbolaser bombardment- Earth, after all, has no planetary shields with which to resist such tactics, nor, indeed, anything even theoretically able to strike back. The Federation presence is functionally a nonentity- unable to meaningfully oppose the Death Star any more than Earth is, simply due to the sheer disparity of scale. Hells, the DS can just grab any impertinent Federation ship with its tractor beams, and blast it at will with ion Cannon and turbolaser fire until it is destroyed or disabled!
As mentioned, a more reasonable balance would be
Executor. Still far larger than any federation ship, but lacking the sheer size which makes the Death Star effectively immune to anything that DS9 has on hand, the massive army that makes conquering an entire planet feasible, and with a commander that probably isn't going to immediately jump to "obey or we bomb you flat" like Tarkin.
