I would suggest that your logic is entirely backwards if you think you're being safer for your "discreet" situation.
The guy who tells you he's undetectable is more likely to be being honest, and accurate, as he's likely to be on his meds and testing regularly. That's because the guy who tells you he's negative may or may not be lying, or may not have any fucking clue because he doesn't test regularly.
Now, if you don't trust the science and the medicine, that's an entirely different problem for you. But statistically speaking, you're less likely to catch ANYTHING from the undetectable guy, because not only is the science clear that undetectable means not transmissible to a statistical certainty, but the guy who's undetectable is also likely testing regularly for STIs too and so is going to be on top of that as well.
Sure, you can still use a condom, and if that helps with your confidence, great. And if you're on PrEP, then statistically you're more likely to get in a car accident on the way to the hookup than you are to get HIV from an encounter with someone undetectable.
So, let's be honest here... this is about your fear, not the science. And your fear is leading you to irrational conclusions. That said, you do you. The undetectable people you're not fucking are probably better off not dealing with your baggage.