We have a static gateway and DNS servers for the WAN connection with several IP aliases port redirected to internal servers. There is now 4 options for connection detection, when there was only ARP detect on the 3900 (But also ARP for LAN)

What is the difference/benefits/drawbacks between the 4?

ARP detect
Ping Detect
Always On
Strict ARP Detect

How often does the connection detection run?

I had no idea what "Strict ARP Detect" meant, so I googled it...
...the first hit in the list, led me to a Draytek.com web page entitled: Difference between WAN Connection Detection modes

See: https://www.draytek.com/support/knowledge-base/4809

Still doesn't explain that much, but I will probably set it to ARP detect or always on.

I would be good to know scenarios of using the different modes, like if certain routers don't like ARP or VLAN'd WANs work best with Ping detection Etc....

How many internet connections do you have

Only 1 , set it to allways on.

If more than 1, then experiment with the settings to see what works with the failover settings you configured.

I will have 2 connections. At the moment there is 1 and I used ARP detect which is working