I may be wrong here - and very happy to be corrected by those more technically minded - but I think there is a way.
In Signal group chats, snippets of your replies to messages are sent to everyone - even if you’ve got them blocked! By this I mean: you can send a message in a group chat, but if someone replies to your message, everyone can see the first line of text (to a degree) even if they’re blocked. So, to rectify this you write something on the first line (emojis, a full stop, any letter etc) then press enter twice - something like this:
MESSAGE HERE
So, the preview shown will only be and not the message. Those you have blocked (or even those not on block) will just see “” in the reply preview. NB: Those not blocked will still be able to click the preview to take them to the correct message referenced)
Why am I saying this here for this use case? Well, I suspect this may also fit in this instance. Apple notifications don’t show you the WHOLE message, just a snippet. So, adopting something similar to the above might work. When sending messages, try making the first two lines or so meaningless. See what comes up on their notification. If it’s garbage in, it’ll probably be garbage out?
EDIT: Incl Spoiler
Formatting the spoiler may also work (but I can’t offer any guarantees as I don’t know whether Apple notifications sees the original message or the obfuscated one)
EDIT 2: After some testing, it looks like the /multiple line method will NOT work. The entire content is displayed in the notifications. So while it’s good for group chats / blocked users, it’s not good here.
BUT the ‘Spoiler’ formatting of text does seem to prevent the real message being displayed. What that looks like on the Apple logs I don’t know….