This works fine, just use the USB-C and Thunderbolt outputs on the G2 for 4K displays.

Basically: No
You can display the same screen on several screens, but you can't expand it.
This is not a problem with the docking station, but with Apple itself, which does not simply allow this. (There are enough discussions about this if you google it) With Windows this is no problem at all and works wonderfully.

My solution: I also spent a long time looking for a good docking that I could connect via Thunderbold 3. Finally, I ended up with the offers from CalDigit. They have specialised in exactly this problem and solve the whole thing with improved software in the docking station.
The TS3 PLUS is great if the workstation is only used by Thunderbold 3 devices.
And the USB-C PRO DOCK is great if the workstation is also used by someone with a non-Thunderbolt 3 USB-C connection.
I use both of them at different workplaces and am very satisfied with a Macbook Pro 2020 16''.
Unfortunately digitec is not a reseller of CalDigit docks. But they deliver directly from the online shop on their website to Switzerland.

BUT: with a Macbook Pro 2015, you don't have Thunderbold 3 yet, so I would recommend you to take 2 remaining Thunderbold 2 adapters and connect 2 screens that way. At least that's how a colleague of mine does it.