Tried the Replace Blocks command through the GUI. Works wonderfully, it just leaves an outline of the area that I replaced blocks, which is distracting

This is intentional as the current block selection will be highlighted. If you find the outline distracting you can disable it in config file. The options is called highlight_blocks

Oh, how do I do that? Sorry, new to modding

For the latest version you can navigate to ..\.minecraft\config\worldhandler\worldhandler.toml and edit it with a normal text editor. In there you find a line with highlight_blocks = true. Change that line to highlight_blocks = false and save the file. There is a chance that the game will instantly update the setting but if not restart the game and the outline will be gone.