I have had problems when sending multiple or commands in a short timespan - already turning a channel ON/OFF within e.g. 2 seconds is hit-and-miss and sometimes produces errors and unresponsiveness. So I would say the Modbus board is probably not the right choice of hardware for short-interval switching.
Generally RTU is (from what I understand) a sequencial protocol initially made for two-wire communication and is thus quite slow.
For precise switching in the millisecond range I’d say you’d have to define the command on the controller itself and then have the ability to trigger that command remotely. I dont think any hardware and/or remote connection will be fast enought to send and process an ON command and and OFF command to a useful level of precision in such a short timespan.
I’d look into Shelly and their onboard scripting capabilities - from what I read, they support sub-second pulsing.