Hi, I would recommend making a cobblestone generator and extending your island. Any valuable items such as, gold blocks, diamond blocks, redstone blocks, and emerald blocks are worth a lot and can help you level up your island by placing them down. If you do all of this and your island doesn't get a higher level, I would type /is level so that way it can update everything. Hope that was helpful, you can always message me with any questions! )
Adding to nicki, one of other best ways to level up your island fast is by getting villagers that gives you emeralds, try focusing on these ones as you can farm emeralds and then you can make emerald blocks which can level up your island as emerald blocks give the most island level points, I would also recommend going to people profit shops, some people may have profit shops on their islands and they are mostly Iron/Leather/Meat but if you wanna focus on your island level you will have to focus on going to the profit shops that offer iron for profit And then you can turn them into blocks which then can increase your island level. If you have any questions please feel free to dm me <3 and have a nice day