Just posting a general comment since there's a lot of replies and just think this would be easier!
I have no interest in terraforming because at the start of the game I flattened my whole island (and regretted it later) -I've tried my best to get it back to how it was at the start from memory so I don't want to change that up! I absolutely love decorating, but it seems like that's all there is to do on the game and that's mainly what I'm struggling with. Atm I'm still doing my town and trying to re-do my house.
It's not that I'm bored with the game or don't have fun I'm just looking for some activity suggestions that aren't related to decorating. Since once of my main struggles is that I can't seem to find much else to do other than decorate.
I think I'll try and figure out what bugs and fish I'm missing from what seasons and go about catching them. Time travel isn't something I want to do so it will give me a reason to play each month. I didn't even realize that there was friendship levels in New Horizons, especially so much so that it changed up the dialogue. A big problem for me is getting repeating dialogue from my villagers so I think that will help a lot!
Thankyou so much for all the suggestions <3
Here are some things you can do that don't involve decorating/"terraforming"- I don't know if you have achieved all these goals, apologies if you have done them!
a) Grow all the flowers
NH has the biggest amount of breedable flowers, with the amount of colors and complex breeding system making it a very nice goal to achieve. The different species have from 6 to 9 colors you can get, and they have have the additional purpose of being resources needed for crafting lots of items.
b) Get photos from your neighbors
You can either get all 10 from your dreamies (if you have them) or make the absolute madman goal of getting all 397 photos. The repeating dialogue you mention may have been related to the weird "order" system this game has, where you need to talk constantly (4-5 times per day) to your neighbors in order to get more "interesting" and "varied" lines- this has been fixed with the latest update, and it's easier and faster than ever to get through the "contextual" dialogue (1-2 times). This also has the side-effect of letting you get requests from them faster. Oh and some reactions are tied to your friendship level, so you may also want to get those.
c) Complete the critterpedia/complete the museum
With 200 different creatures to catch, 55 pieces of art and lots of fossils, this is a gargantuan and fun task (except maybe 100% the art wing as Redd is a bit hard). Many creatures feature different tanks/cages, so it's also nice to collect 'em all; and almost all the diving creatures have tiny special animations when they are inside their tanks! If you want to go even further, there are 160 furniture items locked behind the fishing and bug catching activities so that's another goal you can go for- instead of only needing to catch 1 of each fish/bug, this game incentivizes you to catch 4 of each (5 if you want to keep one with you if you want to re-read Blather's dialogue). Those very rare fish/bugs make this task very challenging.
d) Complete your catalog
There are thousands of items. There are thousands of color variants. You can either go the easy way and catalog everything from other users, or try the hard way of getting almost all of them by yourself. If you go for the second one, be sure to shake your trees everyday to get the guaranteed 2 items and visit Mystery Islands to get other items there. The catalog of this game is not only limited to items bought from shops and items received from special NPC's- this game also has you get all the DIY recipes and the resources needed to craft them. You may have gotten the Blue Rose Wreath DIY and the Golden Casket DIY, but if you don't have blue roses and few gold, you need to get those in order to truly get all items.
e) Complete all the Nook Miles Stamps
This is an activity that takes years to complete. Even if you don't need Nook Miles, you are also getting the new titles to be used for your passport and ymmv if you find satisfying to get all of them marked as complete. Some of the miles are a bit grindy, but you don't need to go full-out grind, you can go at them little by little. Don't force yourself to craft 500 tools on one day in order to get the 3,000 needed- craft 5-10 daily and more when you're feeling it and the progress will go up slowly but surely.
I don't really know what you were doing on NL, maybe there is an equivalent on NH that you are missing. If not, I hope those activities are something you can get behind and enjoy! And like others have said, take a break if you need it.