Update: got an 8mm gasket from Bella Barista only because it is black. I couldn’t face a bright blue or red one popping out from an otherwise stainless steel machine with black handles and levers. They’re cheap enough to order a replacement every 6 months.
I compared the flexibility of the new one (flexed nicely with some resistance between forefinger and thumb) and the old one (instantly broke into four quarters)!
I guess it really was time to replace it.