Hi,
I would like to add a watchdog on my router to detect when my WAN connection goes down, and subsequently try to restart the interface.
I have come across a few different implementations:
Which is the best one? I know some come with LUCI screens, others not..
It depends on your personal tastes...
You can test them and find the one that best suits your needs. 
For example, I preferred to build my own personal "watchdog" that isn't based on "ping," since I have a 4G connection.
watchcat with some minor tweaks discussed here:
Interface restart or router reboot are not always the best options there.
For mobile broadband setups USB reset within a custom script can be recommended as mentioned here: TL-MR6400 V4 - QMI Not working - #14 by AndrewZ
For other WAN types, it makes sense to implement additional logic in a custom script before performing the action. For example: if physical link is up -> restart interface; if the link is down -> no action.
It depends how far the breakage is in network hops and whether restart can repair it. I would not bet on 100x router reboots to glue together optical cable.
You can consider mwan3 and a custom /etc/mwan3.user.