Itch is one of the useful place for the game developers who are planning on building the 2D Games. You will find a lot of tools out there for making the games. You would also find the tools which allows you to edit the map, sprites and map tiles. While searching for some of such tools, I came across one interesting tool.
Tiled Map Editor is the tool that I have found over there on itch. The developer of the editor seems to be active on itch. And has been posting the updates. The editor seems to be pretty handy tool especially when you are using game dev engines like - "App Game Kit Classic".
While using the Appgamekit classic, I realized that there are no GUI tools for editing the maps. And also there are simply no way within the software itself to edit the tiles. They have recently released the GUI tool but that is also paid. And investing much on this classic platform is not really worth it.
If you have already invested into code only engines which lack the GUI tools for the editing tiles, sprites and the other terrain layers. In such case Tiled map editor is a good tool for you for making maps and the level map editors.
It has some of the free assets added into the sample folder. However you may have to find the assets as per your requirement and then import that folder into the project. The saved tiled map can be imported in most of the games so you would definitely benefit as well.
Apart from the appgamekit engine, you can also make use of the output of this editor into the Lua based game engines like Love. I have not tested this for the Game Maker Studio but I hope to test it out later this year if I finish some of my godot engine games.
You can check out the Tiled Map Editor.
We use Tiled for map designing on our game. Honestly it's an amazing tool I highly recommend! The more I learn about it, the more I like it.
You mentioned Terrains, which is a very cool tool in it. I also highly recommend learning Automapping. It's a bit of a learning curve, but the flexibility you get from that feature is unmatched.