Add a command like /persona autoproxy sticky that, once used, automatically proxies every next message as the latest persona used, until you use another one or disable autoproxy with /persona autoproxy off.
Alternatively, /persona autoproxy <name> could also set a persona without sending an initial proxy, and use that persona for every message that doesn't explicitly use a different one, and also can be disabled with /persona autoproxy off.
For example:
No proxying for this message.
/persona autoproxy sticky
p1: This message gets sent as p1...
...and so does this one.
p2: But this is sent as p2.
And this!
Oh, this is still p2!
/persona autoproxy p3
All these messages...
...get sent as p3.
p4: But not this one.
Back to p3 for this one!
/persona autoproxy off
No proxying for this one, either.