Add bootswatch

Is it already working in the bootstrap5:^4.0@alpha version ?

I would guess that the last three items on the list are still not done.

Is there any documentation on how to use a bootswatch theme? I don't see an option to select in the UI

As mentioned in description, work in progress.
Bootswatch was added, the rest requires help and/or financing.

It's pretty simple to do manually.

1) download (and rename? it's been a while) the main _bootswatch.scss to your /scss dir and keep the variables file handy.
2) add @import 'bootswatch'; to style.scss
3) copy over the variables from the bootswatch theme (flat, etc) into _variables_bootstrap.scss
4) add $form-text-margin-top: 0; back into the var scss (the other two changes were in my flat theme vars, you can doublecheck whatever one you're using).

Since Bootswatch is already included in the main theme you can just do something like this to use Yeti for example:

1. In your sub theme "style.scss" add:

// Import Bootswatch theme
@import '../../../../themes/contrib/bootstrap5/dist/bootswatch/dist/yeti/variables';

// Sub theme styling.
@import 'variables_drupal';

// Bootstrap overridden variables.
// @see https://getbootstrap.com/docs/5.2/customize/sass/#variable-defaults.
@import 'variables_bootstrap';

// Include bootstrap.
@import '../../../../themes/contrib/bootstrap5/scss/style';

// Import Bootswatch theme styles
@import '../../../../themes/contrib/bootstrap5/dist/bootswatch/dist/yeti/bootswatch';

// Include custom changes.
@import "custom";

2. Compile the changes like:
sass scss/style.scss css/style.css && sass scss/ck5style.scss css/ck5style.css

Yeah that is a simpler approach.

I just grabbed the bootswatch files I had made for the previous parent theme I tried out before settling on this one. I was at their website to look at the options anyways. :)

Importing from the parent theme is simpler assuming you don't want to change any of the variables bootswatch sets (or think about overrides etc), though that doesn't mean you couldn't just import the theme scss. I liked just having them pasted into my variables file so I could see what has been changed, but that's personal preference.