Would it give me more armor if i would use Lightweight Alloys with Hull Reinforcement Package,

So my question is, "does it really work this way?"

Yes. The bulkheads raise your ship's hull points by a percentage of its base health, while the hull reinforcement package raises it by a flat absolute value, which in this case is higher for the same mass. The trade-off cost is in the highly limited internal module slots.

Specifically, for Military grade composites, the increase is +250% (though for the free lightweight alloys it is already +80%). The Imperial Courier's base armor health is 80, which is raised to 144 (+80%) by the stock lightweight alloy bulkheads. Upgrading these to military grade composites raises it to 280 (by changing the +80% to +250% instead).

The 3D hull reinforcement (as of this writing) gives +260 health for 4t, while the 2E gives +150 health for 4t. These are the only four-ton packages fittable on the Courier, so in different combinations I'm getting 444, 554 or 664 hull points for your second build (maybe things were rebalanced over the years since you got 524).

Still, though, if you are going to hull-tank you might as well go for both the bulkheads and the reinforcement. The 8t increase has only a small impact on sublight speed and jump range.