Error while updating TexLive using tlmgr

I have a working TeXLive 2020 on my Ubuntu machine, that I installed a couple a months ago (previously, was TeXLive 2018, I think).
The installation was done using the "prefered" TeXLive way (not from the OS package manager).

However, I encounter an error on a document that used to build correctly with the previous version. Basically, it can’t find some package (bclogo, FWIW).

So I wanted to install it manually, and headed here:
https://www.tug.org/texlive/pkginstall.html

But the command:

tlmgr update --list

fails (failed) with the message:

(running on Debian, switching to user mode!)
cannot setup TLPDB in /home/myusername/texmf at /usr/bin/tlmgr line 5604.

Checked that line in the Perl file is meaningless:

die("cannot setup TLPDB in $::maintree");

The folder /home/myusername/texmf exist, but is empty.

Ok, so maybe it needs to initialize some stuff in that user folder.
Indeed, with help from this question, I did this:

tlmgr init-usertree

followed by:

sudo apt-get install xzdec

(No idea what that package does…)

But now, the command:

tlmgr update --list

outputs another failure:

(running on Debian, switching to user mode!)
tlmgr: Cannot load TeX Live database from https://ctan.gutenberg.eu.org/systems/texlive/tlnet

When issuing the command multiple times, it seems to try a different mirror/URL every time, but none seem to fit. Although the servers are alive, I checked with a browser.

FWIW, here some others URLs that the command tries, and that fail too:

https://distrib-coffee.ipsl.jussieu.fr/pub/mirrors/ctan/systems/texlive/tlnet
https://mirrors.ircam.fr/pub/CTAN/systems/texlive/tlnet
https://ctan.tetaneutral.net/systems/texlive/tlnet

Any idea what I can do next ?

EDIT: (more technical details)

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/texlive/2020/bin/x86_64-linux::/snap/bin:/usr/lib/jvm/java-8-oracle/bin:/usr/lib/jvm/java-8-oracle/db/bin:/usr/lib/jvm/java-8-oracle/jre/bin

$ which tlmgr
/usr/bin/tlmgr

$ tlmgr --version
(running on Debian, switching to user mode!)
tlmgr revision 39298 (2016-01-07 03:44:29 +0100)
tlmgr using installation: /usr/share/texlive