Edited Apr. 22, 2019 at 12:28 a.m.
What about Kirill Ustimenko?
2018-2019 MHL: 1.78 GAA and .927 SV% with a league record 12 shutouts
2017-2018 MHL: 1.81 GAA and .929 SV%
2016-2017 MHL: 1.74 GAA and .938 SV%
While it's true that the MHL might not be an especially strong league, his numbers are as good as you could hope for. He has the same sort of problem that Rubstov had in not getting time over the older players in the better leagues. I wouldn't be surprised if the Flyers signed him this summer (in fact, Fletcher apparently said he is interested in signing Ustimenko this summer). In the worst case, he can start in Reading and eventually force the Flyers to trade Lyon or else take over for him after Lyon's contract runs out. I think he could possibly end up being better than Sandstrom or Ersson.
In addition, Wyatte Wylie had 11 G and 36 A in the WHL, making him tied for #13 among WHL defensemen in 2018-2019. The difference between #13 and #8 was 4 points. Zamula was #7 with 56 points.
Otherwise, I'm generally agreed with the analysis. The Flyers really need some more forward prospects with top 6 upside. It's not impossible that none of them could end up good enough to replace the likes of Giroux, Voracek, and Couturier. On defense, they need to start stocking up on younger prospects.