Must be lithium you're short of? Those are required for the reinforcement. Lead for foundations and silicone rubber for bulkheads, and all need titanium of course.

I mostly just used reinforcement, but would sometime use a foundation for a Seamoth dock outside the hatch (see first base screenshot, second one has it as well) As long as it's touching another part of the base it will add integrity. I should have used some bulkheads, now that I've seen what they look like. Each time you add or remove a piece of the base the integrity flashes on the upper left of the screen, which I am sure you've seen.

Good to see you're enjoying it. I said give it 10 hours and you'd know. You're probably just a bit past that now eh? It seems you are making great progress.