I agree - Matthews should have been signed for 9-10, and Marner and Nylander for about 7-8 each (probably less as they were all still RFAs).
The first and biggest mistake was signing an unnecessary 2C for $11. That was what not only started the overpayment cycle, but led to the ruin of our cap situation.
Except that when he was signed Tavares was clearly a first-line C and enjoyed full-UFA status so that came at a premium. Back then, he was a proven 35-goal scorer and PPG center on an Isles team that didn't score that many goals whilst also being right in his prime at 28 years old for the 2018 season.
The signing definitely wasn't cheap (slight overpay actually) and it indeed had an impact on how little money we later had to round-out our team, but it isn't the biggest contributing factor to our depth problems, far from it in my opinion.
The crux of the matter is that while Matthews had shown throughout the '18-'19 season that he was a superior player to Tavares and more important to the team, in the summer of 2019 he was also under control for 4 more RFA years when he re-signed with us. A fact that should have massively lowered the AAV of his deal vs. an older and much more proven Tavares.
Dubas should not have caved-in and given him all that money on only a 5-year deal, buying only a single UFA year. Period.
On a 5-year deal, with 4 full RFA years, Matthews shouldn't have gotten more than say $9-9.5M/year.
And following that logic, Marner shouldn't have made a dime over $8.5-9M.
We coddled our stars way too much, and paid for it dearly depth-wise, and on defense/goaltending.