1-- Will a USB-C/Ethernet dongle actually give me gigabit speed or would I need a TB3/Ethernet dongle?
2-- Will WiFi 5 be good enough for the MBP 16" to get gigabit speed if I chose not to go with the USB-C/Ethernet dongle or should I wait for the MBP 16" to get WiFI 6 (assuming that will be a spec bump)?
Gigabit is 1000mb/s which is only 125MB/s... so USB-C can handle that no problem.
But if you were talking about 10gig speeds (over 1GB/s) then that would probably be best with a Thunderbolt adapter.
As for WIFI 5... I have more questions.
What wireless router do you have?
How fast is your internet connection? Cable or fiber?
What devices in your house are you connecting to?
WIFI 5 has a theoretical maximum speed of 1700mb/s... (or 212MB/s)
WIFI 6 has a theoretical maximum speed of 2400mb/s... (or 300MB/s)
However... you will only hit those maximum speeds under the perfect circumstances and with the proper hardware throughout.
Your router is the ultimate decider of your network speed. Do you have, or are you planning to buy, a WIFI 6 router soon?
If so... then you
might approach the maximum speed of WIFI 6 on a laptop. But that's only if your other connected devices can take advantage of those speeds.
And that leads to the next question... how fast is your internet? If you only have 300mb/s internet... WIFI 6 won't make that any faster. You'd be limited by the speed of your internet connection... not the speed of your network.
Also... if you're connecting to a Synology NAS or other network device in your home... those devices could be the bottleneck... not the speed of the network.
I know people are upset that the latest Macs don't have WIFI 6. But you won't notice a speed problem if you only have WIFI 5
If you need a 16" Macbook Pro now... it's probably not worth waiting for the 2021 model just for WIFI 6