Haha the beginning of your post is a lot like me 😂 Last year I started with collecting frogs and then moved on to sea shells, buried treasures, metals and crystals and so on. I had done a couple of collections before that (plants and insects) but didn't really enjoy doing them. I enjoy them more now 😊
Unfortunately I don't have many tips. The one that comes to my mind is that if you have Jungle Adventure pack it is useful to do the jungle explorer aspiration because the reward trait you receive from it helps to find more treasures. Altough it only helps with Jungle Adventure collectibles.
I wonder where has your sim been hiking? One way to find more plants is to travel to the deeper parts of the Granite Falls and search plants and insects there. It may also be useful to find the hermit. I think they have some plants in their garden and some plants only grow near their house too.