Making it to the second round in the NHL shouldn't be considered an amazing accomplishment either if we're being honest. The fact that the Maple Leafs have been so incredibly abysmal during this 17 year stretch in the postseason has lead minimal expectations, and they've proven even incapable of that.
(I read the rest of your post and I commend the work you put into it, but I am only going to comment on the first part).
It is significantly more impressive than in the NBA. There is legitimate parity in the NHL and a hard cap (well, kind of... Tampa threw a wrench in that a bit). Teams are a lot closer in quality than in the NBA, even if there are favourites.
I don't think comparing the NHL playoffs to the NBA playoffs is really a fair comparison though. It is rare to see double digit regular season wins separate teams in the NHL playoffs. Tampa + Calgary in 2018-2019 were the exception, but Tampa had the most regular seasons wins in NHL history that year and Calgary was playing a team that only had 38 wins, which is rarely ever enough to make the NHL playoffs.
In the NBA, between 2014-2015 and 2018-2019 (I can include more years but I will keep it at the last 5 non-Covid years), this is how much the top and bottom teams were separated in terms of wins:
2018-2019: East - 19 wins, West - 9 wins.
2017-2018: East - 17 wins, West - 18 wins.
2016-2017: East - 12 wins, West - 16 wins
2015-2016: East - 13 wins, West - 32 wins
2014-2015: East - 18 wins, West - 13 wins.
Before the Covid years, in which case 2 #5 seeds made the Conference Finals, the last time a non-top 4 seed made the Conference Finals in either conference was 1999 when the #8 Knicks made it all the way to the NBA Finals (but lost). I can tell you that a non-top 4 seed has made it to the Conference Finals on numerous occasions in the NHL since 1999. An "upset" in the NBA is seeing a #4 in the Conference Finals. It's not apples-to-apples. Making the 2nd round in the NBA is probably the same as making the playoffs in the NHL. Winning the 2nd round can be seen as winning the first round, although even then we see far more upsets in the NHL than in the NBA in that regard. The real NBA playoffs start in the Conference Finals.
Should we have made it past the first round by now? Certainly. Should we have made it even further than that? Probably. Dubas obviously feels the same way, which is why he does not want to make radical changes to the team.