In preparing for WordPress 5.0 on behalf of clients I just found the Classic Block. Not the classic editor, but the TinyMCE classic editor inside the new Gutenberg editor.

Tested headings, paragraphs, media, alignments, formatting, insert media and so on, as used to. Edit as HTML.

The whole classic block can optionally be converted to blocks.

Client group 1 is mostly writing posts and occasionally editing pages: If feeling uneasy with Gutenberg and many blocks, show them Classic Block.

Client group 2 is maintaining custom content (post types) with some metaboxes provided by different plugins or own code: Investigating. Maybe turn off block editor for some types. If still problems, activate Classic Editor to see f it helps.

Client group 3 is using a page builder with columns/boxes: Investigating. Most will probably not see Gutenberg, but blocks from the page builder as before. Investigate later if Gutenberg can replace the page builder.

Client group 3 edits pages with a lot of (often nested) shortcodes and never see what they get before previewing: Maybe work in HTML mode for a while until they are all available as blocks and voila - new world! There is now a lot - Atomic Blocks is a good start and a place to look, and some come with core, like columns and buttons.

1. Use Gutenberg and discover what it brings. You may love it.
2. Always use just one block, the classic block. Se easy, and as used to.
3. Install Classic Editor, works at least untiil Dec 31, 2021, probably "forever" in some way.
4. Don't upgrade this major, just minors, at least until the waves have calmed, few weeks or months. Previous majors have de facto long term security updates, so it's not risky.