Firing him around January would have had some creedence, not now months later after praising him for months and Trump finding out that Comey has been investigating him and his campaign for almost a year now.
No one knew of the FBI investigation of the Trump campaign, even former DNI Clapper, until Comey testified as such on March 20th. Trump firing him any time after this date reeks of a coverup no matter the explanation.
Having him write the memo recommending the action isn't really all that strange and goes quite a ways towards explaining the timing.
People would have complained that it was about the investigation anyway in January.