Upgrading Drupal

I first banally launched a composer update that miserably failed with some incompatibility.

Problem 1
    - drupal-composer/drupal-scaffold[2.5.0, ..., 2.6.1] require composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.2.0] but it does not match the constraint.
    - Root composer.json requires drupal-composer/drupal-scaffold ^2.5 -> satisfiable by drupal-composer/drupal-scaffold[2.5.0, ..., 2.6.1].

Then I tried to follow the documentation at the URLs below.

Hi- Hoping someone can help!

I'm trying to upgrade a Drupal 8.8.1 site, eventually to D9, but initially to 8.8.4 (which I read somewhere is a needed waypoint for some module). After backing up and manually moving the new core+ files in place, it crashes with the error "'CommerceGuys\Addressing\AddressInterface' not found". Addressing is a library required for the Address module, which is required for Commerce, etc.

According to the Address module info, manual installation isn't possible, so I tried using Composer to reinstall/upgrade Address, but I'm getting this error:

I have a basic page with images and headers, after switching to CKEditor 5 content is empty when I go to edit the page. The content does load when I view the page.  Do I need to do anything to make it work?

Which version are you currently on?

Is there any blueprint or something that you have?

Apparently I could achieve the upgrade in the subject (the renewed website apparently does work), though I still wonder whether I could have possibly done something wrong.

I'd like to receive comments on the steps described below, hoping in something reassuring.

My recent migration from D7 to D9 seems to have duplicated my content blocks.

I'm currently seeing a top block labeled as id="block-mynewtheme-subtheme-system-main", followed by an identical one called id="block-myoldtheme-subtheme-system-main".