They do overlap but they also don't, CanvasBlocker sees the Canvas and fakes it, then RFP kicks in and randomizes it. So the result is the same as RFP alone but if that fails and gets disabled (by a now unknown exploit) it gets faked by CanvasBlocker so it will still be random.
Like Thorin-Oakenpants (who contributes to Arkenfox) says at the bottom of this
github issue it's fine to use CanvasBlocker in default mode he also talks about a few times in the
Arkenfox Wiki.
RFP is superior and Extensions can leak so having it as a fall back so I can use it to spoof Navigator is more secure than CanvasBlocker alone.
Adblockers are very easy to fingerprint by testing the list entries so adding all the defaults is fine but the more special lists you add (country specific etc.) makes you more unique. If you start adding manual lists you'll become even more unique because the scripts that identify adblockers are written for add-ons such as uBlock or AdGuard and test those lists. By using a separate add-on like ClearURL it will be harder to track because it does things locally as far as I know. But I could be wrong.