[Shinni]

you called

self:SetCustomPinEnabled( pinType, true )

but defined

function CustomMapPins:RefreshCustomPins(pinType)
    ZO_WorldMap_RefreshCustomPinsOfType(_G[pinType])
end

this should be

function CustomMapPins:RefreshCustomPins(pinType, bool)
    ZO_WorldMap_RefreshCustomPinsOfType(_G[pinType], bool)
end

which then results in an attempt to index nil value error in

local drawCallback = function(pinManager) 
            
            -- Get the zone the player is in 
            -- this is how the gatherer addon gets it so hopefully it is correct
            local pinZone = GetUnitZone("player") --GetMapName()
            
            -- Get the existing pin data for the current type and zone
            -- And recreate the pins as we have just redrawn the map
            local harvestData = XMPins.Data[pinType][pinZone]
            for i,v in pairs(harvestData) do
                pinManager:CreatePin(_G[pinType], v.key, v.x, v.y, v.radius)
            end
        end

as XMPins.Data is still an empty table.
you probably want to iterate over SVData.