Let's just say, hypothetically of course, that you are giving advice to a geezer with an antique 357 six shooter What sort of ugrade would provide useful capability in low light situations and not be excruciatingly expensive?
Mostly you should just carry a good tactical flashlight. By tactical I mean easy (for you) to quickly turn on and off one-handed, and possessing significant output. "Significant output" used to mean at least 100 lumens, but I prefer 500 lumens or more. I want my light to have a chance to temporarily blind an assailant, maybe even pee a little.
If you're already carrying a good light, I'd probably prioritize low-light training and practice next. If you're already doing that, you're way ahead of the curve. Tritium night sights work well for me.