Oh, I thought of an alternative suggestion but completely forgot to add it.
Are you aware of the Table of Contents inspector? It works very well with PDFs, but also markdown! If the point is:
and:
This might be a simple, built-in solution. It doesn’t help you with folding sections in the preview – but other users have wished for that, and actually found this a superior alternative. For example:
(which also links to an earlier discussion.)
A screenshot:
It works in the preview as well as source/edit mode. You can navigate it with the keyboard.
It also works with .rtf files, though with some limitations. I rarely use rtf, so I can’t comment too much on that. I remember there was a thread recently about improvements to this, if you search the forum.
… But if you wish to add specific color or formatting to some text, this can also be done in markdown with html tags and/or custom css. E.g.:
<span style="color:red">Some red text</span>
Or
<span class="red">Some red text</span>
with something like this in your stylesheet:
.red {
color: red;
}
To make this faster, you can use a text expander utility or the (more limited) built-in macOS text replacement feature. I have a bunch of snippets/macros set up so that I type 2-3 letters, and then my cursor is magically wrapped in some html, ready to type the actual text 
It might not be for you, but I wanted to make you aware of the possibility. There are a lot of posts in the forum on CSS, if you search around. Or on the wider web.
If that’s too much of a hassle or too complicated, DEVONthink also supports multimarkdown syntax for underlining and highlighting: {++Underlined++}, {==Highlighted==} — for some ways to mark text besides italic and bold.
If you want folding in markdown, you might wanna check out the excellent, free MarkEdit. (It also has a table of contents for navigation, and keyboard shortcuts to jump to next/previous heading. And works with the new Apple AI/language tools, since you’re on Seqouia.) I know this doesn’t help with the preview in DEVONthink, but I mention it as a snappy external editor that doesn’t require you to index a folder like Obsidian. If you decide to use the markdown/ToC inspector-approach, but have a case where you just really want to fold some sections anyways.
