So would it be up to code for me to add another 100 amp breaker for another subpanel to this? Because that would bring me up to about 230 amps potential fault current, which IMO does not sound "slightly" oversized anymore.
Counting up the breaker numbers is completely meaningless, regardless of what the general public might think.
This is a case of 100 + 30 NOT equaling 130.
There used to be a type of panel called a "split-buss" panel. There were four or six main breakers. Typically a 2p50 for the range and lighting main. Ten assorted other 2p breakers for the other "mains". 100A was a typical size. If you counted up all the mains you'd often get over 200A. Again, a meaningless number.
A standard electric water heater is typically on a 2p30A breaker. This appliance will never see over 19A in use.