@Hans
What I am doing with my aiostreams CI/CD, where I automatically deploy a release to both Aminet and os4depot, is to use sed to change a placeholder in the readme templates

For now, I do it on os4depot.readme file (https://github.com/walkero-gr/aiostreams/blob/master/os4depot.readme) where I set on release the OS4DEPOT_PASSPHRASE from a secret variable I have in the CI/CD platform I use

You can see how I do the replacement at:
https://github.com/walkero-gr/aiostreams/blob/master/.drone.yml#L106

and then I upload the release file and the readme, using FTP, to os4depot. The same way could be done to change automatically the version, although that's something I haven't done yet.

- The script would update the version number by reading the current version number from the program (or the *.lha archive name, which has the form
_.lha

That release job is initiated when I create a new git tag, where I set the release version. So, this can be used as the version string inside the files. By doing that, you don't need to get the version anywhere else.

- The script would extract the latest changes from "Release Notes.txt," pulling out the changes since the last upload

That is possible to be done with bash script/line, which can put it again with sed into the readme template. I do not have something like that right now to show you.

- Bonus: upload the archive and readme to Aminet (e.g., using curl)

The CI/CD platform I use is based on docker almost completely. So I use docker containers to do the FTP upload. I think that any CI/CD platform that supports docker containers, and most of them do, can use the same images as the ones I use.

At https://github.com/walkero-gr/aiostreams/blob/master/.drone.yml#L80-L96 I prepare the Aminet release and do the upload, and at https://github.com/walkero-gr/aiostreams/blob/master/.drone.yml#L97-L117 I do the same for os4depot. This works fine everytime.