For people that type a lot using Level 3 (e.g. using the QWERTZ layout), AltGr on Caps is probably more important. On NixOS, it's set with two options:

{
  services.xserver.xkb.options = "lv3:caps_switch_capslock_with_ctrl";
  console.useXkbConfig = true;
}

The kernel console REALLY doesn't like E1 putting the lv5 switch on <M-f>, and instead deletes the f key, so don't use the second option if your layout does that. (Just try it out and don't gc until you've verified all keys work.)