In WordPress/gutenberg#29335 a solution has been developed that make alignments more declarative.
ie A new solution makes it a lot easier to use alignwide and alignfull. I believe this means you no longer need a ton of confusing CSS.

Since SB is a brand new theme I feel I should implement the solution in this theme first.
Later, when comfortable with the solution, and when a run time version of Gutenberg is delivered, I should apply it to Fizzie as well. See http://github.com/bobbingwide/fizzies/issues/44

Requirements

As for Fizzie. Properly support alignwide and alignfull, but with widths as below

Solution

• Switch to using a newly built version of gutenberg-source
• remove assets/alignments-front.css - I'd already done this
• Change experimental-theme.json replacing "custom" with "layout"
• Implement `{"layout": {"inherit": true }} attribute on relevant blocks in templates/ template parts
• Try using different blocks with different widths.

Which is basically the same as for WordPress/theme-experiments#233

Caveat
I don't know what to expect with the sidebar.