Your experience is almost the opposite of mine. I find the square and rectangular metal tins to be the most dodgy for long term storage, and by that I mean more than a decade. The square and rectangular tins aren't really 100% sealed from the get go. They're slowly leaking, and you can prove that by filling up a Tuppeware container with those tins, sealing the top and leaving it for a week or so. Then pop the top and take a sniff and you'll smell tobacco. There's no mystery to this. The pressure of the seal is uneven around the perimeter, mostly concentrated at the corners. These tins were never meant to hold up for years and years. I've had a number of failures of these tins over the years.
On the other hand, the round screw down tins, because the pressure is even around the perimeter, last quite a bit longer, as do canister tins. Put those in a Tupperware container, seal and leave for a week, and when you pop the top there's no tobacco scent.
Regarding the use of Mylar vacuum sealed over tins, if it isn't metallized Mylar, it's not going to prevent the contents from drying out over time. The reason is that clear Mylar is permeable, while metallized Mylar is not. If you're thinking of cellaring for a couple of decades, use metalllized Mylar. Currently I'm enjoying some 22 year old Haddo's Delight that was packaged in metallized Mylar, and it beats the tins from that era, many of which failed due to corrosion, or left a trace metallic note.
Solutions all depend on just how long you think you will want to cellar something.