Yeah, battery acid is sometimes used, and not a terrible corrosive risk with the precautions you note. I don't love that it doesn't come with any specs e.g. on heavy metal contaminants, but I think it's near-certainly fine in practice. ACS grade would have specs, but I don't want to handle the concentrated acid.

Sulfuric acid would give me excess sulfate, but since the plants aren't too sensitive to that I expect that would be an improvement over my current excess phosphate. I'm not sure if I'd get calcium sulfate precipitation as the reservoir concentrated. Nitric acid would let me hit my target profile exactly (with a corresponding decrease in nitrate salts), but it's way more dangerous. I've seen nitric acid used in academic trials and in countries with weaker safety laws, but not commercially in the USA.