Hi there! I'm using Joplin 1.8.5 (prod, darwin, revision: b28f087), and I'm running macOS Big Sur 11.5 Beta (20G5023d) on a MacBook Pro (Retina, 15-inch, Mid 2014 — 2.8 GHz Quad-Core Intel Core i7, 16 GB 1600 MHz DDR3).
I mostly use the Rich Text Editor almost all the time (switching back and forth to Markdown when I occasionally need to 'fix' some formatting — fortunately, a rare occurrence). As you all know, the Rich Text Editor's toolbar is not quite the same as the one for the Markdown Editor (even assuming that we're only using Markdown features or plugins that work on both).
I'm pretty sure that everybody has their 'favourite' toolbar setup (possibly based on other tools that you use). In my case, for instance, I use Heading 4 (e.g ####) quite a lot (and I use it every day, without exception); the Rich Text Editor conveniently has buttons for H1, H2 and H3, but lacks H4. I'm sure that very few people will need H4, so this is not really a feature request... all I need to know how I can change my toolbar.
I've looked on GitHub and these forums for a simple, straightforward answer, and my searches resulted in nothing (I admit I may not typing the exact combination of keywords that will get me an answer!). I'd even assume that there would be an option under Preferences to change the toolbar(s) layout; or, at least, that some kind and generous volunteer might have created a plugin for that.
However, I seem to be unlucky — I haven't found anything so far 
Now, as far as I can read from the source code, this Rich Text Editor is nothing more and nothing less than good old TinyMCE (I've come across a message in a thread where @laurent vaguely suggests that in the future there might be a different rich text editor integrated in Joplin). I'm a bit agnostic towards rich text editors — so long as they work and do their job well, I'm fine. TinyMCE is the rich text editor used by WordPress, so it is a very solid and reliable editor, having been tested by a billion people — or at least hundreds of millions — so naturally enough all questions I might have related to Joplin's Rich Text Editor can actually be 'solved' by tinkering with TinyMCE.
Changing TinyMCE's toolbar is quite simple — if you're willing to hack the source code. That's a solution if you really need an extra button or two on the Rich Text Editor toolbar. Of course, these hacked changes will disappear with the next version update — meaning you need to hack things every time you update Joplin...
Since I'm not acquainted with the source code of Joplin, I wonder if this is not simply an option or preference that can be set 'somewhere', I just didn't come across the documentation which explains how (or where) these changes can be 'safely' made (i.e. persisting from one update to the next).
I understand that the Rich Text Editor is 'experimental' in all regards and that I should not expect it to mirror faithfully what can be achieved with 'pure' Markdown. Nevertheless, I like WYSIWYG. I'm just bothered about the lack of an easy way to change the toolbar buttons, that's all.
I suppose that, if I were just a little more proficient with Jolin's code, this functionality could be very easily implemented and submitted to Joplin's repository as a pull request, and refrain from asking a newbie question here on the Support Forum...
Anyway, as said, pointers to a potential solution/fix are most welcome!
Thanks in advance,
— Gwyn