I'm not sure which instructions you are trying to follow there, but that step doesn't seem right.

As far as I know, the partitioning tool used in the installer will expect that you first set up raid member partitions, thereafter you set up the raid device itself, and thereafter you partition and format that as if a single device.

1) "Normally" a migration is done with the running OS, and in that case it's really a VT console level activity.

Last time I looked into it, the Debian system seemed to have some new layers of confusion with a series of (Windows-ish) steps to  go through to reach a VT console level. That might be different now. The important bit is of course that the migration script needs to not be terminated prematurely when the init transition occurs, and therefore the shell that runs it must not be under any "session process control".

In your case, you could instead hook up drive B as the disk for a qemu virtual machine, and then migrate within that while idling in front of the browser on the "main" OS, drive A. The only particular quirk with that would be how to make drive B bootable by qemu (perhaps using extlinux)

In all cases you'll need a final grub setup after the migration; i.e. running update-grub with OS A to let it (re-)discover the drive B OS.

Though, I think I would call that a 201 activity as it does require sllghtly more than notional knowledge about running a qemu virtual machine, and about equipping it with an extlinux boot. It might not be your ideal solution but that's the way I can think of to let you view the instructions while following them. Printed on paper might be a better option.

2) You will need to re-update grub with OS A. Possibly already at the first of the two migration reboots.

3) The pinning will be preserved and shouldn't cause any issue ... unless, well, unless something.

4) And it shouldn't be an issue with keeping the old Tk either.

Why not pick one of the installer ISOs instead?
They have desktop, server or netinstall in their names.
https://www.devuan.org/get-devuan

Is anyone (still) getting those warnings from

wget --no-check-certificate https://www.devuan.org/get-devuan

The certificate was updated 2 weeks ago, and is valid some 8 or so weeks more. So there shouldn't be or have been any warnings.

No, nothing special or strange.

You post the link for the paste here, and then anyone can follow it  while it's valid.

Note that the certificate was Issued On Friday, October 21, 2022 at 10:55:06 AM and it Expires On Thursday, January 19, 2023 at 10:55:05 AM, so something strange is happening here.

Please do the following to download the certificate complained about

echo -n | openssl s_client -connect www.devuan.org:443 -servername www.devuan.org \
    | openssl x509 > /tmp/wwwdevuanorg.cert

and then upload that to pastebin.debian.org for us to review.

maybe you can teach your browser something by adding  .html to the end instead of /

which link?

It all happens in the partitioning step:

1. first you declare partitions for use in raid setup

2. then you set up the raid device linking it to its partitions

3. and then you partition that into the filesystem partitions you want

"installing the base system" at 6% is where packages are loaded and validated; nothing yet unpacked.
If it breaks there, then it's due to one of 3 things:
  1) that your media is "corrupted"/broken or
  2) that the target partition is too small or broken, or
  3) that the RAM (which holds the installer system itself) is too small.

libsemanage-common_3.1-1_all.deb is a mere 20K, and loaded after maybe at about 30Mb has been used on the partition.

Unless you make a mistake when partitioning, it would suggest that your media is corrupted.

The published ISO image does not come with that corruption (as evidenced by that it's the same image since start and more than a few people have used it successfully)
You need to verify your download of that image using the published SHA256SUM, and you may well need to verify the physical media it is placed on as well.

Ok;
.. so I choose language (english), location (US.. lying), locale (en_US.UTF-8), no additional; just "continue"
.. I skip past access software
.. I skip past speech synthesizer
.. I do configure keyboard (am english)
.. it skips selecting layout automatically
.. I let it detect media
.. I load the default components only (select nothing of the extras, then "continue")

are we same enough so far?

So you have difficulty with the chimaera netinstall iso? Do you want assistance?