There is no way to allow external messenger access without at least some metadata.

Yes there is. Lots of them do. Some of them don’t even have servers to collect the data in the first place.

Even Signal allows a server to collect metadata to a small extent.

Which is completely unnecessary and also close to zero.

That’s not what we’re discussing.

I’ll take “Facebook has metadata about my messages” over “WhatsApp is continuously running in the background on my phone 24/7” any day of the week.

What is it you think it’s collecting outside of metadata?