Now that my WordPress installation has been updated to version 5.8, I've decided to install an updated version of the Seedlet theme that I've used here previously, alongside the Gutenberg plugin, and try out full site editing.
Initial impressions are fairly positive — it has been fairly straightforward to remove some things that are superfluous since I'm the only author and don't use categories or tags.
One quibble so far is that it's not immediately obvious how various elements are groups together within each template and template part — an outline guide would be very helpful. I will need to dig into the Gutenberg settings and see if there are any things I can change.
Another oddity I've noticed is that there's a discrepancy between how posts display in the editor and how they appear on the site — it seems that the font settings in the Seedlet theme aren't been applied when editing? I'm not sure what's going on there.
There is definitely potential here to drastically improve the ability to customise site appearance while reducing the code required for themes. I have a feeling that things won't gel together until WordPress 6.0 at least. For now, this is very much an optional feature, and I suspect most sites will continue working just fine with traditional themes while people figure out what Gutenberg and full site editing could offer them.