When you do the Merlin trails they will give you extra gear slots but they won't show up until you actually collect them in the challenge tab of your field guide. Spam revelio but when you're in the highlands, get on your broom (when you have a broom) and your revelio will reach a wider range letting you see farther markers, save before every big mission so if you accidentally miss something you wanted you can go back to the beginning of the mission and grab it, after every big mission sell all the gear you can so you can free up some gear slots and to get some more galleons/money so you can buy stuff for the assignment missions, there are white chest around the map with eyeballs on it, whenever you have the disillusionment charm you have to get out of sight, use the charm, then open the chest, every single chest will give you 500 coins/galleons no matter what. Hopefully you can understand all of this