Little wonder that playing a character who was little more than a sipher - and to be fair to the writers of MI, most tv characters were at that level in the 1960s - was boring after his Spock years (or ears).
Actually M:I was worse in that regard. Look at the very early episodes, the first half or so of the first season, and you'll see the characters have more personality. We see more of them being themselves, either during mission prep or when the mission goes wrong and they have to break character. We see them trading banter and dialogue that gives them personality. We sometimes see them having doubts about their ability to pull off the mission. In "Old Man Out," we actually see Dan Briggs (Jim Phelps' predecessor) trying to convince a reluctant old flame to help him on the mission, the one time we ever saw the recruitment process. Sometimes we even saw conflict within the team, as in "Memory" where the other team members doubted the ability of an alcoholic team specialist to do his part, or in "The Short Tail Spy" where Dan suspected Cinnamon of losing her objectivity and falling for an enemy agent. At that point, it was about on a par with the level of characterization you'd typically get in a '60s show. You didn't get deep into their innermost souls, but at least you had a sense of who they were when they weren't roleplaying, and they had a texture and humanity to them.
But by the second season, and arguably the end of the first, all that had been quashed. Bruce Geller decided he wanted to focus solely on the missions, and almost never had the characters break character outside the apartment briefing scene. That's when they became ciphers, far more so than was typical for the era.
I suspect Nimoy was drawn to the role because it let him play a different character every week, since Paris was always pretending to be someone else. He probably welcomed the opportunity to stretch his acting and emoting muscles after three years learning to be as reserved and internalized as possible. But maybe eventually he got tired of the fact that Paris had no personality of his own, or maybe he just came to find the stories too formulaic and repetitive despite the chance to play different characters.