Using an external timer to synchronise start times
Setup with external timer (top left), video score (top right) and Zoom (across the bottom) for Bastard Assignments' synchronised-start piece Fugue in c minor BWV847
This approach can be used with any other tool or combination of tools, screen space and devices allowing.
The simplest form is to use a clock which is centrally controlled in some way, which could be using a phone clock (reportedly most reliable if all performers are using the same type of device, e.g. all iPhones or all Android devices) or a service such as timeanddate.com, as in this image. Agree the time to start, then at that moment, all begin, regardless of what you can see or hear from the other performers.
Another possibility to consider is Chronograph, a cloud-synchronised stopwatch, which one person can start for everybody.
This approach can be combined, as in the image above, with video or audio scores or clicktracks.
Note that this will reduce the desynchronising effects of latency, but will not eliminate them entirely.