For anyone else looking at this- I just did an extensive search to see if TinyMCE (Advanced) can be used with HTML / source code. I’ve found a number of other similar inquiries on the topic, with no solutions. The maintainers of that fine software are adamant that TinyMCE is for editing front-end content, not server-side code. So it’s not in the scope of this specific tool to facilitate editing code. That was a bit of a shock for me but I’ll go with whatever the author says.
There is a hacky way around this.
I believe you’ll need both the Classic Editor Plugin and the TinyMCE Advanced Plugin. On installing the Classic Editor, go to Settings>Writing where you can allow all users to toggle into this editor (I’ll leave related issues to be solved outside of this info.)
Open a post and add a few blocks.
From the three-dot menu switch to Code Editor. You can also use Ctrl+Alt+Shift+M.
You’ll see the HTML source with embedded Gutenberg comment tags.
Copy that source code.
Open that menu again and under Plugins, Switch to Classic editor.
Now you’re back in familiar territory. But do not use that text.
Paste the source code from your clipboard buffer into the page. You’re replacing the output-formatted text with the source.
Modify that code as you wish using find/replace.
For more sophisticated editing, don’t bother jumping back into Classic, open a real text editor app or IDE.
Do not save that page – although there’s no harm if you do. You are not interested in saving the markup underneath the text that you are editing. You want the raw text.
When done editing, again copy the text in the entire editor area. (Ctrl-A may not work – use the TinyMCE menu to Edit>Select All.)
You should have a new metabox in the side of your page. Go to Editor > Switch to Block Editor.
You’re back to Gutenberg. Cancel the offer to restore from an auto-save, if it’s there. You should still be in Code View mode.
Paste your code. There should be clean blocks delimited by comment tags with a single line break between each. If you see markup that you did not add then the copy/paste operations were done incorrectly.
From the three-dot menu switch to Visual Editor. You can also use Ctrl+Alt+Shift+M.
You should see everything exactly as you wanted within each block.
I doubt many people will go through this rigorous exercise, but I hope it helps anyone who chooses to do so.