Absolutely love that you threw it back to your previous header design. That was the hope in this test - that folks would revisit what they once tried and feel the differences. It makes me excited thinking about what things will feel like a year from now :) Now to address your various items of feedback as is our ritual!

The problem area is the Columns block used for the Site Logo, Navigation, and Buttons across the top. You might as well hang up any hope of that working well on smaller screen sizes.

For anyone following at home, you can follow along with the work being done around responsiveness for these exact problems in the following issue: https://github.com/WordPress/gutenberg/issues/34641 The screenshots in the issue nearly match what you show above.

I hit no spacing-related problems in this experiment. That feels gratifying to say after over a year of testing FSE features.

Woohoo! Most recent addition on this front is having margin support added into the Group block: https://github.com/WordPress/gutenberg/pull/37344

The Navigation block may be my least favorite thing about the site editor. I have yet to see how it will offer a universal system that plays well with the 1,000s of design variations that theme authors will want to employ. Classic nav menus are still vastly superior for custom design.

While not everything will be solved ahead of 6.0, I did want to note this tracking issue for the navigation block items currently: https://github.com/WordPress/gutenberg/issues/38275 In particular, I like seeing what the priority items are vs what will need more time to mature (to better set my own expectations). I agree that the navigation block has a ways to go to catch up to the many, many designs we've grown used to.

Users can add a background to the entire Navigation block but not to the individual menu items. How did I do it? I used a Buttons block instead.

I added this excellent feedback to an issue around adding separator options to the navigation block: https://github.com/WordPress/gutenberg/issues/23293#issuecomment-1055680205 I also want to note the desire to have the buttons block in navigation block as yet another way to accomplish some designs we're used to: https://github.com/WordPress/gutenberg/issues/35924 I imagine you could "hack" it with the row block but that means it wouldn't show up in the responsive menu among a host of other issues.

WordPress’s mobile Navigation menu icon provides no customization options. Users can customize the overall background and text color but not target the button directly. I faked it a little by narrowing down the column.

Noted in this issue around both having options to style the responsive icon and to use your own/have a few options: https://github.com/WordPress/gutenberg/issues/37930 This is one I'm particular interested to see since I use FSE on my personal site and find the current icon doesn't feel distinguished enough (want to mess with it heh).

Twice during testing, my Navigation block disappeared on the front end. I suspect it had something to do with me trying to adjust the outer Column block’s width. However, I was unable to replicate the issue at will.

I haven't noticed or heard anything like this but will now keep watch for more/similar reports. I appreciate you noting things like this even when you can't fully replicate. There have been quite a few times now where hearing about items like this have led to discovering very real issues so know it helps!

The experience of designing from within the site and template editors feels pretty smooth these days. The holdups are more about missing capabilities than anything.

Hear, hear. Now that the building blocks (pun intended) are in place, I find myself regularly ready for more options along with wanting more intuitive flows. Things have come a long way and still have a long way to go.