This would 100% depend on the extension. He’d really have to sign cheap or Philly would be in trouble

We’d give up our best RHD prospect, a shot at an elite RHD, and probably not be able to re-sign Sanhiem or land Johnny Hockey to do this

I hear what your saying
Attard may be Philly's best RD prospect but he is by no means an elite prospect, in fact I couldn't find him on an top 100 NHL affiliated prospect list over the last 3 years, so I'm guessing he's probably a B to B+ prospect. The value I keep hearing for Miller is a A to A+ prospect, a 1st and a young NHL roster player, so I think value wise this is probably pretty close based on how high the pick is.
Of course the trade would completely depend on the direction Philly chooses to go this offseason. It would make no sense for them to part with young assets if they aren't planning to push for the playoffs next season, or if they don't think they will be able to regularly compete for the playoffs going forward.