It looks like you're using base64 encoding. If WhatsApp allows an extended alphabet then you might be able to switch to base85 for a slight performance bump.
Since WhatsApp is end-to-end encrypted you can probably just send bibary data. Stick a prefix on it so that the real client is guaranteed to ignore it as corrupted.
I think the only risk is that if you have a real client running it reports the invalid messages and WhatsApp uses this as a signal to van your account.
The E2EE here is not about privacy, but about being able to send whatever data you want (like binary) since WhatsApp will only see one type of data (encrypted) in transit, in contrast to needing to send data in a specific format to have it transferred at all. Meta can peek at the original "messages" all they want, they will see encrypted packet data anyway.
I think the only risk is that if you have a real client running it reports the invalid messages and WhatsApp uses this as a signal to van your account.