[Baertram]

But there is no way to tell addon1 to do this and addon2 to do that without implementing this to "each of those addons themselves"!

So no, there is no "addon" which helps you to output chat messages of other addons to dedicated chat channels or tabs. This would only work if the one addon aiming for that goal would overwrite the other addons' chat message handling in total (and most likely will break them or any compatibile addons this way).

Addons can use LibDebugLogger to add info, warning or debug messages behind a dedicated addon related "short key".
LibDebugLogger will log those messages then and you can use the extra UI addon DebugLogViewer to see and filter the addon related messages beside the chat (in a small quick logegr UI or a bigger UI with filters).

Side effect: All addon d() (debug messages) which normally are shown at the "system" chat channel won't be shown in the normal chat anymore but at the DebugLogViewer UI then!

But this also needs those addons to implement LibDebugLogger and it's messages, at each addon.