He's better than Reinhart when Reinhart was traded (1st + decent prospect)
He's better than Garland when Garland was traded (other stuff was in the deal, but a top 10 pick was the get)
He's better (and younger) than Pacioretty when Pacioretty was traded (really good prospect + 2nd)
He's better than Zucker when Zucker was traded (1st + good prospect)
Fiala should get a prospect of Suzuki's quality (1st line/1st pair upside) + a 1st in the 15-25 range, or a 1st in the 5-15 range + prospect of Addison's quality (2nd line/2nd pair upside).
I agree with everything you said except the Garland comment. The only reason ARI got the 9th overall pick is because they also traded OEL (with some retention) and took back a bunch of bad contracts in return, to a team with a GM who was desperately trying to keep his job.
And the other caveat with any Fiala return and whether it's a big package or not, is whether he agrees to an extension with whatever team is trading for him -- which I think is the most likely scenario because teams would be much more willing to move significant pieces if he's going to be there long term, and MIN would want to maximize their return.
But again, I don't think they trade him. I think they try to keep him and move out other pieces (Dumba?) instead.