Ideas:

1. hub included in PMs

the intermediary solution "my hub gets included in every private message I send" would reduce the load on phone being forced to decrypt, and create known vulnerability for reading messages BUT mean I could not have my key stolen

2. a new sort of private message

• leak to: meta-data to reduce the load on account that you know are on phones
• keep text body encrypted
• e.g.

  // msg.value.content
  { 
  type: 'envelope',
  to: [ '@+UMKhpbzXAII+2/7ZlsgkJwIsxdfeFi36Z5Rk1gCfY0=.ed25519' ], // @andre-mobile
  text: 'asdasdlhxinsdxjkajalkdxkjahdsx' // private message for andre-mobile + others?
  }
  

• the to: field is only for recps who have declared they are phones. This means that here only andre's phone knows to bother trying to decrypt. Other desktop recps might also try it out and could have their meta-data as recps preserved.
• you could obfuscate with dummy to: recps (like pick some other random users)
• when andre replies to mix-desktop, he can use classic private messages, so not everyone is taking a privacy hit. He knows to decrypt his own messages ...

the more I think about (2) the better this idea seems ... it's so simple and not so terrible

The message type is envelope because you can see who's it's too but the contents are hidden...