Hey Robert,
Interesting question. I haven’t customised the template for any of my Discourse communities but I’ve spent time researching email subjects for mail drops from Mailchimp etc.
The ones that I find do the best aren’t capitalised (so they don’t look like titles). Ideally they’d be personalised but that’s not feasible in this case so I’d say that you’re likely on the right track with at least removing the square brackets. You probably need to be careful though because if someone writes a PM with a spammy looking title (“help me please”) then that might affect both delivery and open rates.
That said, people are going to get used to recognising these at a glance regardless of what they look like so I’m not sure you’ll have a huge impact on your numbers.
It’s probably a case of testing a few options out.