I think any Flex fare qualifies, no? At least that's my read of Altitude website. I'm also E35k.
"With the required number of eUpgrade Credits, access the comforts of Executive Class on flights operated by Air Canada and Air Canada Express with the purchase of Latitude or Flex fares. Sun destinations are defined as those within the Caribbean, Mexico and Central America."
Perhaps? I use the tool for checking how many eUp's are required to upgrade with E35 status from YYZ to GRU and it said Flex (M) for 12 eUps.
This past year, as Elite status, I needed 17 eUps but the Tango Plus fares had more classes than just M.