I'm thinking to compensate my light meter reading with -2 stops for the ambient light and then set the flash power to normal for the model. Rear curtain sync is no option in this case.
It would be easy to set the exposure balance if you use a handheld light meter that reads ambient light for the background and flash for the exposure on the model. First, determine the aperture to get the model exposure by flash, then using that aperture setting, meter the ambient light for the time needed for the background exposure.
Shoot at dusk when there is still enough daylight to see the streetscape and set the exposure of the streetscape a stop or two down. Then adjust the flash to fill-in the model.
PS the fill in flash exposure is probably the more tricky to get right, it might be a good idea to bracket expose the flash in multiple frames.
go check out Neil Van Neikerk's site. He's got LOTS of instruction for just this very situation.
Basically you set an exposure for the background then drop two stops...then dial in the speedlight to make it look natural and not lit.
An incident meter is very useful; not only will it help here, you will get real numbers to remember and not "just a bit more" which is pretty subjective.
If you're shooting with off camera flash, you control your DOF with your aperture, your lit portion with your flash power/flash distance from subject, and your ambient light with your shutter.
For example, set up your composition with your model and and set your aperture to the general DOF you're going for with that lens/FOV, then you change your flash power/flash distance from model to until model is correctly exposed, then, dial down your shutter speed from the sync speed until your ambient exposure (everything the flash didn't light) starts to come up. From there it's just a matter of how much ambient exposure you want.