just use epsom salt (formula non-standard, but which has the benefit
of being something I already have.)

Well, it is 0-0-0 but that's only because those three numbers only hit
3 of the nutrients, not the other dozen or more things which plants need.

Epsom salt provides magnesium (only). So it is apples and oranges
compared with something like triple phosphate - you might need both,
or neither, or just one. Won't harm anything, at the recommended
dosage, though.