A laser driver (MLD203P1) with an external Bias-Tee for modulation
Have you looked at the Thorlabs T1G bias-T PCB? I use these with constant current drivers to modulate DFB lasers and superluminescent diodes. There's a decent set of instructions linked on the product page - you attach it to the output of the driver and then to the device you want to drive/modulate. It will definitely do the modulation bandwidth you are after. I can't get the link at the moment as their website is down.
I'm surprised that Thorlabs didn't suggest this to you. From the previous comments - are you expecting to modulate in constant power mode? You need to be using the driver in constant current mode and then the bias-T superimposes the modulation waveform on the DC current.