You could use the “Search & Replace in Current File” command to search for - [x] and replace it with - [ ]
If you wanted to automate it even further you could use a program like AutoHotkey to create a macro that would do the same thing but require less or no typing on your part.
You may be able to find a plugin that already does exactly what you are looking for. I’m sure someone will let you know if that’s the case. Otherwise, you could just begin browsing them in the Obsidian plugin browser dialog or somewhere like Obsidian Hub.
In terms of the separators, you could try a css snippet. I found this one thanks to this post, "How to achieve" CSS code snippets ,which links to a GitHub repository full of helpful snippets:
div.markdown-preview-section hr {
margin-block-start: 30px;
margin-block-end: 40px;
}
Or maybe you could just use a different theme.
Good luck!