Currently at 5/32" on all 4. Do I just wait until ...
Two aspects, IMO, are what one needs to use as a guide: first, tread depth; second, the actual grip of that tread (irrespective of the depth).
If it were me, at 5/32" I'd start looking and evaluating options.
Invariably, Over 35+ years of driving on multiple vehicles, multiple different tires, I've found the grip levels to plummet
long before the actual tread depth demanded it. But, that's with my tire selection, my driving style, speed, routes. Not everyone's going to have the same experience. But, definitely, consider both aspects.
At 5/32", I'd certainly start looking. Prior to the next rainy season, myself I'd definitely swap in a new set.
But, generally speaking it's the grip itself that drives my choice. If on normal colder or wetter mornings, prior to the tire fully warming up, I find the grip to be noticeably down from peak, I'll get seriously aware of grip under all conditions. Once a greater range of situations begins showing such reduced grip, that's it, at which point I get a new set. For me, at least, given the roads I often drive, selecting marginally-grippy tires is a luxury I can't afford. More expensive to do it this way, I suppose ... but then, a deadly crash is a bad way to save some cash.
