USB-C dock to add an extra monitor irrespective of GPU external limit

@Luke_Filewalker  Dock pundit here.   If you have a 13" MacBook Pro that only has Intel Graphics, then that GPU only supports 3 simultaneous independent displays, counting the built-in display.  On Windows, the built-in display can be completely disabled in order to run 3 external displays, but that isn't possible on Linux, so it might not be on macOS either.

In terms of your question, the D6000 uses DisplayLink for the first two displays that are connected -- neither of which would count toward your GPU limit -- and then if a third display is connected, whichever display is connected via HDMI relies on USB-C DisplayPort Alt Mode, which DOES count toward your limit since that taps into the native GPU output on the USB-C/TB3 port.  (If you use a D6000 with a system that doesn't support that, the D6000 can only run two displays.)  However, be aware of DisplayLink's potential drawbacks, which I wrote up in the post marked as the answer in this thread.

Additionally, be aware that the D6000 only provides up to 60W to the attached system.  Depending on the specific MacBook Pro you have (always a good idea to provide basic technical info like that when asking for technical assistance, fyi), that might be less than what the system is designed for.  In that case, you may experience slower battery charging and/or reduced performance unless you keep the system's own power adapter connected as well.  You'd have to choose which of those options is preferable to you.  The WD19 dock family can supply up to 90W to attached non-Dell systems, but they rely on native GPU outputs, so they wouldn't help you sidestep the maximum display limit.

@Luke_Filewalker  The D6000 is primarily a DisplayLink dock, so the only way you’ll get any GPU-driven display with it is if you have 3 displays connected to the dock, and then you’d only get the HDMI display driven natively. If you want use some GPU-driven displays, then you’d want a dock like the WD19TB — or the CalDigit TS3+ or CalDigit USB-C Pro Dock, both of which are popular especially with Mac users. Any of those when used with a Mac would allow up to two GPU-driven 4K 60 Hz displays, and then any ADDITIONAL displays you wanted to run could use DisplayLink USB dongles connected to the USB 3.0 ports on the dock itself. That might give you some flexibility in terms of which displays are used for which content based on how different displays are driven. But the D6000 is essentially just a pair of DisplayLink USB dongles stuffed into a hub, so displays connected to the D6000 would behave essentially the same as what you observed with USB dongles (except for possible differences due to specific DisplayLink chipset and/or driver version). But if you go with the WD19TB, take a look at this article: https://www.dell.com/support/article/en-us/sln320770/dell-thunderbolt-dock-wd19tb-and-apple-usb-c-hosts?lang=en

DisplayLink lag seems to be based mostly on how much is changing on the displays, however many there are. For example, having 6 DisplayLink displays all showing basic productivity apps where relatively little area of the display changes at any given time would probably be fine. Having even 3 displays playing high resolution video running full screen, where the entire display area is changing constantly, might be tricky. Display resolution would also come into play, since more pixels is more pixels.

And lastly, all of this traffic is running over USB 3.0, so if you’ve got something like a USB 3.0 hard drive or especially an SSD also connected, then you might notice display issues when if you have a large file transfer running. If you can attach that sort of device to a port actually run by a different USB host controller in the Mac, then that would help, but I don’t know how its USB data interfaces are wired, or even if they have more than one USB host controller chip. But if I had to guess, your best chance of success would be to have your USB 3.0 HDD/SSD connected to a port on the opposite side of the Mac from the one carrying your DisplayLink traffic.

Good luck!