Oh, duh. A friend's comment made me remember that for the broccoli soup, instead of adding a stock cube, I stirred in a teaspoon of Marmite. I know, I know, most people find this stuff awful, but it really does add a lot of umami and not a lot of salt. Like fish sauce or anchovies, which you also might not love, you totally can't taste it, but it lifts the entire dish. I'll take a natural solution to the problem any day.