The firewall is a good defense for the hacking computer in a PvP type scenario but in PvE the enemy and neutral factions won't be using a firewall. At best, they'll have turrets that may try to kill you or destroy the computer. For a bit more balance against "defenseless grids" the hacking computer should become damaged over time (burning out computer components) so if a grid is very large you're going to be doing lots of manual repairing or setting up a welder next to the computer. I don't think damage should necessarily occur on failure to hack. Burning out components seems like the cost of hacking regardless.

I would say something like the Private Sail should require 1-2 repairs to fully hack the whole thing (not sure how many computers are in the Private Sail but 1-2 repairs seems like a fair cost to take over something that size).

Not sure how this would manifest itself in the mod. I don't think a permanent change is warranted because in PvP the balance would be heavily in favor of defenders. Maybe have a new workshop item or a variant with a different design (so we don't accidentally place the damage-free version).