Thanks to everybody. I will look for another tank but I now suppose that my tank is fine, is just that it can occasionally be treacherous "by design", as PE and others have confirmed, sometimes it looks tightly close but it is not. It is nice to know that presoaking does only good to the treatment, also with E6. It will also eliminate any risk of killing a roll.
I received an advice by PM about greasing with petrolatum the outer upper lid of the tank to ease correct placing of the fingers, which will also certainly help.
Regarding the difficulty in placing a 135/36 roll into a 1520 plastic reel, the film I spoiled was the easiest one I loaded, and I checked after the problem that it actually was correctly loaded. What I did was to make a very "round" cut on the corner, just like the factory one at the end of the leader, on the other side of the leader. Film insertion was decently smooth.
I'll buy another 1510 or a 2510, the one I find first with a cog.