Except the so-called huge, slow moving target won't be that easy to disable, let alone destroy.
The combined fleet effort may be enough to bring something like the Fuhrer into a position where it won't easily escape, but the losses for the fleet will be very high, possibly losing several important ships, including capital ships if they end up in the gunners crosshairs, and 16" armor belts withstanding a 30"+ projectile? Impossible to avoid. Although even if the huge shell falls short it would still cause immense damage to the enemy ship, possibly getting hit underwater and breaking in half from the force of the projectile hitting and detonating.
And if the Fuhrer did end up coming into contact with elements or nearly whole of the British Home Fleet then the losses would be in favour for the Germans as the Fuhrer would also be armed with additional 15" guns both for naval warfare and fire support.
Take one of the British or American capital ships for example, if it was hit by a shell from the 30" guns then it would have the same fate as the Hood, blown up into pieces or in two.
The thing with torpedoes is, their size is partially restricted to the size of the submarine and the aircraft itself, as even a torpedo the size of the Tallboy Earthquake Bomb, it would suffer from depth problems, such as being too heavy to remain at the required depth level or too large for even operational use, and the Germans would be aware of the vulnerability of the stern equipment, so it would be possible to add additional 'collapsable' honeycomb hull structures, as these take and absorb the brunt of the damage and leave the inner hull unharmed.
Even so for a ship the size of the Fuhrer, its speed would also have to be enhanced by several fractions by the insanely powerful engines the Fuhrer would possess, possibly accelerating to a top speed of somewhere around 35/40knots+, for a short time to preserve fuel.
The combined efforts of entire fleets also brings the dangers of having that fleets' primary target having a strong well defended escort, so the attacking fleet would have to be forced to retreat to avoid certain destruction from both the escorts and the Fuhrer itself. Think about this.
Another thing I forgot to mention, there is a very fine line between being capable of getting brought down and being too big to be destroyed. The Yamatos were under that line when designs were completed, so they were done and dealt with. The H44s on the other hand would just be on the opposite side.
The other thing about being so big is the size of the weapons you can carry, as you said the Fuhrer (H44 class) was designed around the Shwere Gustav and Dora superguns, and having eight of those guns is by far capable of causing very serious physical and psyhcological damage to the enemy, and even more if it had sister ships working cooperatively together.
Even so, if the Fuhrer was on routine merchant raiding missions like the Scharnhorst and Gneisenau and not penned up in fjords like the Tirpitz for example, it would certainly be capable of forcing the British to be extremely wary of the Fuhrer or its sisters when the British ships went on naval patrols.
And even it had sister ships they would be also capable of very intense damage to the Allies' morale in ship losses, multiple defeats and many more by each H44 class ship. This would also add the H44 class to be the most powerful, largest, and the most deadliest weapons ever if they were built.