[Ethereum] Problem installing Ethereum via apt-get on Debian Buster

I want to run a geth fullnode (with fast syncmode) to be able to use the JSON RPC. I have a VMWare-based vm set up which fits all the requirements as far as I can see. Domain. Fixed IP address. Disk space. RAM. The works.

Now this is a Debian system on Buster (10) on kernel 4.19 on an emulated x86_64. uname -a gives you Linux hostname 4.19.0-13-amd64 #1 SMP Debian 4.19.160-2 (2020-11-28) x86_64 GNU/Linux

Now I want to install ethereum via apt-get to be able to have it automatically updated instead of having to do this always manually. I encountered some problems installing the sources, as the ppa-keys for whatever reason weren't imported. Anyhow, everything is set now:

$ ls -l /etc/apt/sources.list.d/ | grep eth
-rw-r--r-- 1 root root 144 Jan 11 17:19 ethereum-ubuntu-ethereum-hirsute.list
-rw-r--r-- 1 root root 140 Jan 11 17:04 ethereum-ubuntu-ethereum-hirsute.list.save

and

$ apt-key list
/etc/apt/trusted.gpg
## truncated
pub   rsa1024 2014-02-10 [SC]
      2A51 8C81 9BE3 7D2C 2031  944D 1C52 189C 923F 6CA9
uid           [ unknown] Launchpad PPA for Ethereum
## truncated

which I achieved via sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2A518C819BE37D2C2031944D1C52189C923F6CA9 (key ID from the launchpad site) and now my apt-get update works without any errors:

$ sudo apt-get update
## truncated
Hit:5 http://ppa.launchpad.net/ethereum/ethereum/ubuntu hirsute InRelease
## truncated
Reading package lists... Done

Yet, when trying to install ethereum (or the packages geth, ethereum-unstable, or so), I get the message "unable to locate package":

$ sudo apt-get install ethereum
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package ethereum

EDIT: Also followed this here: https://linuxconfig.org/install-packages-from-an-ubuntu-ppa-on-debian-linux, but with the same result: "unable to locate…"

Anyone knows how to solve this?