Ever heard of the SUBSCRIPTION LICENSE MAX ? might inform those clients of that or add it to what you charge
Dongles can get lost or damaged.
JMO

Not a concurrent subscription , rather a one time for that system
If a client has a full system a Max license should not be a obstacle for them i would think and not put a burdon on the hired operator

Since when is a possible alternative and opinion a complaint ?

Then again the mention of deactivating their device every week and client suggesting to use trial license (dodging buying ?) does makes me wonder why they dont have their own license in the first place. MAX license was added to cater for occasional shorttime use!

I fully understand your worries when using your license on a foreign system. Maybe i read it wrong but it sounds like its often the same client(s)/systems when this occurs ( as mentioned before JMO)

I used to have software using a dongle and personally found it impracticaly in the long run even without having to use it on foreign machines.

I am not even sure that its economically and practically feasable for vMix(Martin) to provide them and adapt vMix's current security to a dongle system especially as a regisstered key is linked to various installed hardware items and system settings (try changing the language settings for example on a registered machine) .
If my assumption is wrong that i am sure vMix will consider it with enough +1's