So here is my suggestion..

Wouldn't it be better if instead of having 4 instant refreshes + an automatic refresh every 20 minutes..

We get a 4 instant refresh + 1 extra attempt to refresh on well (each 20 minutes)?

This case each person will still have to wait same amount of time but not missing it, increasing the chances to find SS convoys also increasing the chances for plunderers to find SS plunders (Specially in new servers or not so populated ones).