The most efficient way to generate -5V out of 5V3 at 30mA
It's not really complicated to generate -5V out of a positive rail; as you've noted, inverting converters are a thing!
So, a non-isolated inverting converter is the way to go – these aren't inherently any less current-capable than an isolated converter, so there's something about your presumption that it'd be better to use an isolated converter that's not quite right.
Also, &quot;a couple of opamps&quot; doesn't sound like a lot of load at all, so probably pretty compact (and easy!) designs would do.
You can try out a lot of vendor tools for DC/DC converter design. I like TI's webench. I just hacked in Vin= 5.0 to 5.3 V, Vout = -5 V, Iout = 0.5 A, and got this within seconds (along with a few other proposed designs):