Unfortunately, as you have rightly noticed, it is not possible to leave the conversation if there are some in the group who do not use iMessages, for example Android users.
This is because the Android user sends normal SMS messages, which cannot be managed in the same way as iMessages can. So you receive normal text messages, whether you want it or not. The only solution is to block a number, but that might be a bit too drastic.
You have done what you can by disabling notifications from that group, also it is just a matter of hoping that the group shuts down by itself. Alternatively, you can write and ask the group if they can start a new one where you are not part of it.