I have always been a fan of Eheim for canisters. I have several of the older Pro II line. I had a similar problem with the big O-ring that relates to the huge pump button designed for priming. I almost never use it. After I clean a canister I refill it with water before I reconnect it. I am able to lose the intake and return lines when I disconnect them from the lid. This leaves them full of water. When I return the filter and reconnect it, I just plug it back in and it is good to go.
A couple of years ago one filter developed a similar problem to yours. The gasket needed is normally not a part users can replace, But how to do it was on YouTube and Google. So I bought the gasket, followed the step by step instructions and it still leaked. That filter was replaced with a NIB one and the leaky one became a source of spare parts. I have no idea what it would not seal properly, But it was a running filter on a tank so I simple took the top assembly from another filter and put it on the bottom of the broken canister and that way It was still fully cycled and good to go.
But I had an advantage here. I had two NIB canisters I had grabbed a few years back on Ebay. By then they had been discontinued by Eheim who had come uop with 2 model lines since the Pro II. My oldest 20236 has now been going over 20 years. The first decade it double as a CO2 diffuser as I put the output of my pressurized CO2 system into the Eheim intake. It was why I chose Eheim as I did not believe other brands could handle the gas going through it.
My feeling is that given all the things one might have to replace in a canister, the pump O-ring may be the hardest to replace successfully when we try to get it done. I suggest you contact Fluval for suggestions. Fluval was acquired some time ago by the Hagen Group.