6 monitors, one laptop, one docking station

dangerousbeans,

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Short answer: To my knowledge, the only laptop on the market today that MIGHT support 6 displays is the 15" MacBook Pro.

Long answer:
I'll assume you want to use the default 60 Hz refresh rate in all cases rather than dropping to something more sluggish and that you want to use them as independent displays rather than using mirroring.

Running 6x 1440p displays would require the bandwidth of 3 full DisplayPort 1.2 outputs, and I'm not aware of a PC laptop from ANY vendor that offers more than 2 full DP 1.2 outputs for external displays.  The only exception is the 15" MacBook Pro, which offers 4 full DP 1.2 outputs via its 4x Thunderbolt 3 ports, but that's not exactly a PC.  If you've only got 2 outputs of bandwidth, the most they could support would be 4x 1440p, with the exact way to connect them outlined below.  You could look into using USB to HDMI/DVI adapters for the remaining 2 displays, but those adapters use DisplayLink chips, and displays connected through those do not perform as well as displays connected to native outputs on the PC. I'm also not sure any of those adapters even support 1440p, although even if they did, running 2 of them simultaneously might be a problem because you might run into USB bandwidth limitations. However, if you want to know more about them, I can elaborate a bit. (Note that what I just said does NOT apply to USB-C display adapters, since USB-C is a completely different animal in that it has a real DisplayPort output wired to it.)

2x 4K displays will work on most if not all Dell laptops that have Thunderbolt 3 if you use the TB16 Thunderbolt Dock (NOT the regular WD15 USB-C dock), or else a Thunderbolt 3 to Dual DisplayPort adapter if you just need display outputs rather than a full dock.

For the 6x 1080p and 6x 900p scenarios, 2 DP 1.2 outputs is enough bandwidth to carry all of that, but you'd have to split them 3 displays per output.  In terms of how exactly to do that, if the displays in question support daisy-chaining by including a DisplayPort OUTPUT, you can connect the PC's output to Display 1, then Display 1 to Display 2, then Display 2 to Display 3 -- and repeat that for the other output for the remaining 3 displays.  If your displays don't support that, you'd need to buy a pair of triple-port DisplayPort MST hubs and connect one of those to each of the PC's DisplayPort outputs, then connect the displays to the hubs.  Of course this requires your laptop to have dual DisplayPort outputs available, which takes us back to needing the Thunderbolt Dock or Thunderbolt to Dual DisplayPort adapter mentioned above. One advantage of the MST hub option is that you can use DisplayPort to HDMI/DVI/VGA adapters if needed, which is NOT possible in daisy-chaining, which requires that all displays in the chain use native DisplayPort.

All that said, bandwidth is only half of the story.  There are also GPU limitations on the total number of supported displays, regardless of how much bandwidth is available.  Laptops these days typically only support 3 because that's what Intel GPUs can handle, and even most laptops that include a separate GPU still have their outputs wired to the Intel GPU, with the other GPU working as a render-only device.  There are some Dell Precision models that have a "Special Graphics Mode" BIOS option that allows the separate GPU to directly control the display outputs in order to expand connectivity options (with the tradeoff that the separate GPU then runs constantly, which takes a battery life toll), but I'm not sure if even they can do 6 displays since I'm not sufficiently well versed in that particular spec item of various Dell models, and sometimes that spec is even tricky to find.  I typically only see people running 3-4 displays total, with 4-display scenarios only when they include the built-in panel.

If you want to run 6 external displays of ANY resolution, I think there's a very good chance that you'll have to look at a desktop that includes a high-end graphics card, because it may not be possible on any laptop available today, at least not without USB DisplayLink adapters that will not give you a great experience.  And 6x 1440p just isn't possible on PCs today purely for bandwidth reasons (again, ignoring DisplayLink solutions), except for MAYBE the 15" MacBook Pro, which offers enough bandwidth, but I don't know if it will actually do 6 displays.  For every other laptop that only offers dual DisplayPort outputs, that won't provide enough bandwidth to support 6x 1440p until DisplayPort 1.4 arrives, but again you may still face GPU limitations.

Oh, also, on DP 1.2, I should be able to run 60hz

In addition to the above, I don't think your bandwidth figures are taking blanking time into account.  A single DP 1.2 output can only support 2x 1440p @ 60 Hz. If you can't be convinced of that by the numerous sources on the Internet that will tell you so, then try driving 3x 1440p @ 60 Hz off of a single DP 1.2 output and let me know how that goes for you. Now consider that DP 1.3 doesn't even double 1.2's bandwidth, so there's no way a single DP 1.3 output will support 3x 1440p @ 120 Hz, except MAYBE if both source and display support the new Display Stream Compression standard that was just introduced, but your current displays won't.

Also, DisplayPort measures bandwidth in Gbps.  GHz is a frequency spec that's more commonly used on HDMI and DVI.

It's true that DP 1.4 doesn't offer more bandwidth than DP 1.3, but Thunderbolt 3 currently only offers DP 1.2, which does carry less bandwidth.  I just don't expect DP 1.3 to be implemented at this point because that had been the plan for TB3 at launch but didn't happen, and now DP 1.4 is on the market, so I fully expect the next Thunderbolt rev to include it, especially since it adds support for things like HDR.  Anyway, a single DP 1.2 output can 4K @ 60 Hz, as I explained, and you have two outputs available in Thunderbolt 3, so dual 4K @ 60 Hz is possible.  Dual DP 1.2 is also enough bandwidth to handle up to 8x 1080p @ 60 Hz if you don't encounter any other limitations.  However, dual DP 1.2 is NOT going to get you 6x 1440p.