All your questions seem to be about how to calculate if termination should be used or not. This is an XY problem. Simply always terminate it so there is no need to worry if it works without termination.
Yes, standard transmission line rule is to terminate with cable impedance. On the other hand, some systems that use RS485 always specify to always use a ...
You can answer a lot by putting an oscilloscope probe at each end of 200m of cable (several turns around the lab) and seeing for yourself.
My guess is : yes you'll need termination.
see cable datasheet;
test as above;
some leeway (characteristic Z is broadly independent of line length;
DMX512 can support that many devices. Its main use is for stage lighting control. It’s very, very primitive, relying heavily on manual setup to define devices and addresses (no self-enumeration or discovery.) Uses RS-485, and has a backhaul path that’s not well regarded. Nevertheless it’s popular owing to its simplicity and stage-friendly cabling.
That said, ...