I will say I bought some cheaper carts that leaked, but they were only held together by a rubber ring.
After learning that lesson the hard way, I bought Amigo Liberty V9 carts, based on a blog in the Wax Liquidizer website.
They are easy to fill because the heating element comes out of the glass tube. They have a fill mark displacement fills the cartridge when you put it back together. they screw together to limit leaking & also have adjustable air intake, which I haven’t tested enought to have an opinion on.