Does this replace vanilla inventory behavior?

No. It adds armor-grid inventory-bonus equipment and leaves the vanilla character prototype alone.

Is this safe for existing saves?

Yes. Add it to an existing save and research the new harness tiers normally. Before removing the mod, unequip the harnesses and clear the extra inventory space.

Does this affect achievements?

Factorio disables Steam achievements when mods are active. The mod itself does not use cheats or runtime commands.

Does this require Space Age?

No. It targets Factorio 2.0 base game prototypes and uses vanilla science progression.

Does it work with other inventory mods?

It should stack with other equipment or technology bonuses because it uses Factorio's native inventory-bonus equipment system.