I'm shooting models with a D750 handheld, and an SB-800 a few feet away with an umbrella mount.
I've been using the built in CMD mode on the D750 the SB-800 flashes along with the camera, but it is not reliable. Sometimes the SB-800 doesn't see the light and doesn't trigger.
Also, does this setup work with HSS?
The Commander system does work with HSS, however, the internal flash does not.
So this specifically means in the Commander menu, the group for the internal flash must be disabled, with Mode "---". If internal flash is enabled, it cannot allow HSS, because the internal flash cannot do it. Disabling it will still flash commands (before shutter opens), it just cannot contribute lighting if HSS. Groups A and B however will work remotely with HSS, assuming the remote flash has HSS.
You said umbrellas. If indoors, HSS is the poorest and least suitable mode you could choose. No other way to say it. Regular speed light mode will run circles around HSS, in power and speed. 1/250 second shutter speed is more than enough indoors, and will keep out any normal indoor ambient. See
HSS Auto FP - What is it?
Assuming indoors, the commander should work OK with the umbrellas. The flash head rotates on the body, so on the umbrella stand, the body should be turned so that the red side infrared sensor (by battery door) is aimed at the commander in the camera. The head can still point into the umbrella, but the sensor should aim at the commander.
I am puzzled as to "why" or "how", but if the Remote flashes are close to the commander, maybe within 4 or 5 feet, they appear to trigger by proximity. They can be behind or at the side of the commander and they still flash, when you could not imagine they would. But further needs line of sight.
Normal portrait light has the Main light up to about 45 degrees around toward the side the subject (for the lighting). That should be plenty of angle so that flash (with body rotated) should be in clear unblocked plain line of sight of the Commander. The Fill light is back by the camera, very frontal, as close as possible to the lens axis, but so that the lens can still see around that umbrella. The Fill light should be about -1 EV lower than the Main light for lighting ratio. This is set in the Commander menu.
See
45 degree Portrait Lighting Setup for this idea of Main and Fill light.
Commander TTL (is TTL BL) seems to normally need about +1 EV compensation, which means there are two ways to do that.
Flash Compensation +1 EV, Main group 0 EV and the Fill light -1 EV.
Or Flash Compensation 0 EV, Main group +1 EV, and Fill 0 EV.
These are identical settings. The +1 is instead "as required", whatever it needs, but I'd bet it will be ballpark.
The SU-4 does NOT add TTL. SU-4 only works for flashes in Manual mode. However, what it does do (might conceivably do) is that the SU-4 unit can watch the triggering flash, which can be TTL, and it will trigger, and then quench off when the TTL trigger quits. If the flash is near maximum power level, the TTL preflash will trigger it and expend all of its energy too early, but at lower power levels, there will be some left for the final flash again. The Nikon flashes are sort of unique in that they can flash multiple times before recycle, if they have power left.
So that is slightly like a flash exposure control, all flashes quench off at the same time. The camera TTL system does see all the light and meters that total sum and quits when total is sufficient, so that's good, however this has no control of the individual remote units power levels or distances, so the remote flashes are not metered individually. Level is a manual flash thing left up to the photographer.
The SB-800s have SU-4 mode in them, you can play with it to see this, and it is a very good (very sensitive) optical remote trigger for manual flash, but as "TTL", it's an obsolete system today.
So this is nothing like Commander TTL, where each flash (each flash group) is actually individually metered, and is controlled individually. Day and night different.