OK, so I read on another forum that I may try to run "sudo apt remove appstream" instead.
And I did that, and it seems to have worked:
davvy@davvy-Inspiron-3646:~$ sudo apt remove appstream
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
chromium-browser-l10n libaudio2 libcroco3 libdouble-conversion1 libegl1-mesa libfcitx-qt5-1 libllvm6.0
libllvm7 libllvm8 libmng2 libmysqlclient20 linux-headers-4.15.0-66 linux-headers-4.15.0-66-generic
linux-image-4.15.0-66-generic linux-modules-4.15.0-66-generic linux-modules-extra-4.15.0-66-generic
python3-asn1crypto qtchooser qtcore4-l10n
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
appstream apt-config-icons gnome-software
0 upgraded, 0 newly installed, 3 to remove and 145 not upgraded.
After this operation, 2,663 kB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 281953 files and directories currently installed.)
Removing gnome-software (3.28.1-0ubuntu4.18.04.15) ...
Removing apt-config-icons (0.12.10-2) ...
Removing appstream (0.12.0-3ubuntu1) ...
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for libglib2.0-0:amd64 (2.64.2-1~fakesync1) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for desktop-file-utils (0.24-1ubuntu2) ...
I then went ahead and tried the update again:
davvy@davvy-Inspiron-3646:~$ sudo apt update
Hit:1
http://us.archive.ubuntu.com/ubuntu focal InRelease
Get:2
http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
Get:3
http://us.archive.ubuntu.com/ubuntu focal-updates InRelease [106 kB]
Hit:4
http://archive.ubuntu.com/ubuntu bionic InRelease
Get:5
http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:6
http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Hit:7
http://us.archive.ubuntu.com/ubuntu focal-backports InRelease
Get:8
http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:9
http://us.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [31.9 kB]
Get:10
http://us.archive.ubuntu.com/ubuntu focal-updates/main Translation-en [14.0 kB]
Fetched 511 kB in 3s (193 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
146 packages can be upgraded. Run 'apt list --upgradable' to see them.
davvy@davvy-Inspiron-3646:~$
I went on to do the following command, as suggested:
$ sudo apt autoremove
..and then attempted to resume the upgrade again:
davvy@davvy-Inspiron-3646:~$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
There is no development version of an LTS available.
To upgrade to the latest non-LTS development release
set Prompt=normal in /etc/update-manager/release-upgrades.
davvy@davvy-Inspiron-3646:~$
Edit: I checked one more time to see if the upgrade was done properly:
davvy@davvy-Inspiron-3646:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04 LTS
Release: 20.04
Codename: focal
davvy@davvy-Inspiron-3646:~$
I will check around to see if there is any remaining problem.
Do you have any other suggestions to clean up or diagnose whether everything is working properly?
Thank you for your kind help, in any event!