[FlatBadger]

The way you have your code right now means that it will only change the font each time the Player Activated event is triggered, which only happens each time you zone in to somewhere.

If you want it to also change as soon as it's toggled, you need to refactor your code to something like this:

function RidinDirty.AdjustFont()
	local fontBoost = RidinDirty.savedVariables.fontBoost
	if fontBoost == "enabled" then
		local fontString = string.format("%s|%d", "EsoUI/Common/Fonts/Univers67.otf", "26")
		SetNameplateKeyboardFont( fontString, 1)
		SetNameplateGamepadFont( fontString, 1)
	else
		local fontString = string.format("%s|%d", "EsoUI/Common/Fonts/Univers57.otf", "20")
		SetNameplateKeyboardFont( fontString, 1)
		SetNameplateGamepadFont( fontString, 1)
	end
end

function RidinDirty.PlayerActivated()
	RidinDirty.AdjustFont()
end

-- Nameplate Font Boost Toggle
function RidinDirty.NamePlatesToggle()
   local fontBoost = RidinDirty.savedVariables.fontBoost
   
   if fontBoost == nil or fontBoost == "" or fontBoost == "disabled" then
		fontBoost = "enabled"
		RidinDirty.savedVariables.fontBoost = fontBoost
		df("|c9900FF[RidinDirty]|r FontBoost: %s", fontBoost)
	else
		fontBoost = "disabled"
		RidinDirty.savedVariables.fontBoost = fontBoost
		df("|c9900FF[RidinDirty]|r FontBoost: %s", fontBoost)
	end
	
	RidinDirty.AdjustFont()
end

As for the events, I only unregister the addonload event during the addonload - one of my addons uses many, many events, none of which I unregister at any point after that.

function BS.OnAddonLoaded(_, addonName)
    if (addonName ~= BS.Name) then
        return
    end

    EVENT_MANAGER:UnregisterForEvent(BS.Name, _G.EVENT_ADD_ON_LOADED)

    Initialise()
end