Hmm, but on my Sony XA2, on that final window (which I had trawled through), the very bottom line (under Access point names) says:
To disable this SIM, remove the SIM card.
This is in contrast to what was possible under the older Oreo-based version of /e/, which allowed a click to disable a SIM (but not both at the same time, so one or other had to be on; that was OK).
Therefore, if this is this something that is available on the OnePlus, where did it go for the XA2?
Hopefully it will make a return with the new S-based eOS that is coming soon to a Sony XA2 near you!