For future reference, in the end here are the steps that worked for me, from a book bought on ebooks in the adobe DRM format, to a Kindle Scribe:

to prepare Calibre:

• install Calibre
• install the KFX Input plugin from inside Calibre
• download the DeDRM plugin (https://github.com/noDRM/DeDRM_tools, specifically, download the latest stable version linked in the readme, it should be a .zip folder inside a zip folder, uncompress the main one to access the DeDRM_plugin.zip) and then add it from inside Calibre, selecting that .zip folder

to download the book (specific to eBooks)

• buy the book on eBooks
• follow their instructions to download the Adobe Digital Editions (ADE) software 🙄 and "authorize" your computer (you can choose the option which doesn't require creating an account)
• download a link to your book
• do "open with" on the downloaded link which is in a weird format and select ADE (you might need to find the path to it, likely in Programs / Adobe / etc) to add it to your ADE

to convert the book

• in ADE, right-click on your book and select "show folder" to see where is the PDF
• in Calibre, do Add Books and go to that folder to select that bookNow if the correct plugins have been installed, the DRM should be removed in the process of importing to Calibre.

to send the book to a Kindle Scribe

• go to Amazon send to Kindle website and login to your Kindle account to send the PDF to your scribe in a format that allows annotations.
• wait a bit, and if your kindle is connected to WiFi the book should appear!
• another option is to send it to your kindle via Calibre (best if you can connect via USB) but it will either be in PDF format, which doesn't seem to allow annotations / scribbles, or converted to a more kindle-compatible format, like AZW3 or KFX (another plugin needed for this) but this ended up being poorly-formatted in my case.

Overall, yes, this was a pain and I just wish you could do with books like GoG does with games to be used in any platform of your choice and share with people if you want, like we'd do with books or games before, in a reasonable manner because of course we want authors to be paid their fair share.

Useful links:
https://www.ebooks.com/en-uk
https://support.ebooks.com/hc/en-gb/articles/206061173-Reading-your-ebook
https://www.reddit.com/r/Calibre/comments/1c2ryfz/2024_guide_to_dedrm_kindle_books/
https://ewritable.net/why-you-cant-write-on-some-pdf-files-on-the-kindle-scribe/

@elduvelle You can definitely strip the Adobe DRM using Calibre (see the instructions at https://ebookconverter.blogspot.com/2020/10/remove-drm-from-adobe-digital-editions.html for details), and I think you can probably convert it to a Kindle friendly format at the same time.

You might also like to know that the book is cheaper on the Kobo UK site, You'd still need to strip the DRM etc, but it's another option - I use Kobo for almost all of my ebooks these days, but don't use a Kobo device or app.