So, I dreamt of your island and I would definitely start placing down paths. I think for cottagecore, the natural Path custom design would obviously work well. You'd have to explore that area and what type of look you want since there is now tons of different variations of it. It would help figure out what little nooks you want, how you want trees to obscure or wrap around sections. Terraforming helps a lot too with filling space and corralling the player along.
On your map, I roughly scribbled ideas down. If you have an art program or you can just use paint; this helps to visualize how you want others and yourself to navigate the island. Your original map, on the right side had this long, straight pathway to the secret beach and I found it not exciting. You had this heart pond, and I thought why not blow that up bigger. Make it more special. It can be a magical forest there. A villager could live there. A lot of possibilities.
I'm also not a big fan of giant fruit tree orchards. Your museum could be there or anything really. I think you could get away with dispersing your fruit trees across your island. It would just help add variation and color since we only have 3 different trees. But if you prefer your fruits trees there, then it's fine too.
I think your airport is a good start and you could follow through with more of the ruined pillars and the other pieces in the set. I could see the whole bottom left be romantic ruins and it can slowly transition to something else.
Maybe for this picnic area, could be obscured by more trees, so it has that cozier aspect. You could even use cliffs to enclose it more. Think about if you want a main path leading here or a hidden path. That's why I think paths are really the foundation to building up your island.
It's just one of those things you have to just jump in and start exploring. It's just going to be a lot of trial and error. One program I used a lot when I was beginning to design my island was the
Happy Island Designer. You can upload your island map into this and draw over it.