I was shooting in a fairly dark room with two Yongnuo YN560 IV off camera in softboxes and a Yongnuo YN560-TX II on the camera to trigger them. I was not able to focus without having someone hold a flashlight on the model until I got focus and then remove it before the shot. This very annoying, because it requires an assistant. Is there any product in the ballpark of a thing that mounts on a light stand and is just essentially a flashlight that is remotely triggerable? So you could hit a button to turn on the light, get your focus, hit another button to turn off the light, then take the shot?
Cheap option. Attach the flashlight to a stand or your camera and shoot. The speedlights will overpower the flashlight anyway so it won't be visible in your photo unless you are shooting at very low power.
Point being that you don't have to worry about turning the flashlight off before you shoot, just ignore it and shoot anyway.
Many camera mounted speedlights and controllers have near-infrared AF assist lamps. They illuminate briefly when the shutter is half pressed and go out once focus is confirmed. Such AF assist lights require communication between the camera and transmitter (or flash) that use the proprietary pins for each brand that surround the standard center pin on the hot shoe connection.
Your Yongnuo YN560-TX II does not include an AF assist lamp.
The newer YN560-TX PRO transmitter does include an AF assist light and would allow you to control your YN560 IV flashes while also giving you a camera mounted AF assist light. The YN560-TX PRO would also give you the ability to control YN622 system receivers that have been updated to the latest firmware versions (using either manual or TTL power control).
If you are otherwise happy with the results you are getting from your speedlights and softboxes, $55-60 for a new controller is a lot cheaper than switching over to studio lights with modeling lamps.
If you want to be able to control studio lights remotely the way you can control your YN560 IV flashes (power, zoom, groups, etc.), you'll either need to choose from whatever limited options Yongnuo offers or buy another controller that matches whatever brand of studio lights you buy (assuming that brand even has remote power control as an option). It seems that Yongnuo's only studio strobes with remote power control capability use the YN622 protocol, rather than the YN560/605/603 protocol.
If you decide to go the studio monolight route, Godox is probably your best option. Their entire system of manual speedlights, TTL speedlights, bare-bulb speedlights, and studio flashes all use the same communication protocol.
Since the YN560-TX II works for everything other than the "wake from sleep" function via the center pin, there is another workaround that should give you what you need if you are willing to give up the "wake from sleep" function.
You should be able to stack any controller with a "pass through" hotshoe and an AF assist light between the camera's hotshoe and the YN560-TX II. One such trigger would be a YN622. If you use the version for your camera brand (Canon, Nikon, etc.), which would be required to get the communication needed for an AF assist lamp, you may even be able to preserve the "wake from sleep" feature.
So you could hit a button to turn on the light, get your focus, hit another button to turn off the light, then take the shot?
Using a studio strobe, you typically have a modeling lamp. You don’t have to turn off the modeling light to shoot. They’re enough light for you to get an idea of the lighting and focus. But the strobe flash will greatly overpower the modeling lamp such that your proper exposure, if the strobe didn’t fire, wouldn’t record anything at all. Those lamps aren’t that bright.
Copy the idea if using speedlights...get a dimmable bulb on a floor lamp and give yourself just enough light to focus. Don’t worry about turning it off, it’ll be greatly overpowered.