Euwe was very fortunate to become WC -- as Alekhine played several WC games against him drunk...........Smyslov, Petrosian, Spassky and Botvinnik -- as Botvinnik was fond of saying -- the WC is first among equals............and so, those 4 -- though very strong did not have an aura of invincibility.
I like your list of the strongest ex WC as Tal, Karpov, and Kasparov -- all very active, dominating players. Bobby FIscher was not active as a former WC -- so you rightfully left him off your list.
Carlsen will soon be the strongest ex-world-champion ever - the player who is strongest even after losing his title.
What are other strong and successful ex-world-champions? The following come to mind:
Other ex world champions were less successful. Steinitz and Lasker played chess for a few more years after losing their title, but ended their competitive career soon. Aljechin died. And let's not talk about Bobby Fisher.
What about others? How strong were Euwe, Smyslov, Petrosjan and Spasski after losing their title?