Well if it’s specific collectibles it would make sense to do a parent class “Collectible”, then do your specific collectibles.
Place those in your level and then whenever you pick up one (wherever you decide to handle this) do a “Get All Actors Of Class”. This will provide you with an array of all actors of a specific class. If you select your “Collectible” class there it will select all child classes so as soon as the length of the resulting actor is 0 you should do your level end routine.
Another way would be doing the same via the level blueprint. Simply reference all actors and bind “On Event Destroyed”. If it happens set a boolean in an array or check somehow if all actors are collected. If so end the level.
The first solution is more dynamic but requires you to create a new actor class for every collectible while you could use everything in the second method.
Cheers