The Big Bang Theory airs all the time on TBS (and is heavily in syndication), so it's already getting plenty of exposure. Who cares if it's on Nick at Nite or not (I haven't watched Nick at Nite in the last few years)?
I am surprised to hear that they are already adding reruns of Young Sheldon, when the show is still in production (have they added other shows that were still in production? Not counting The Best of Saturday Night Live). I didn't see much of season two, would be good to see those episodes (I also feel like I don't see the first season of The Big Bang Theory often and don't remember much of it). With the last season cut short, a lot of season 3 episodes were rerun a lot (I feel that episode where the mother wants Sheldon's sister to have a baseball trophy where the player's head is clearly a girl was rerun a lot, I recall seeing it about three times), I kept thinking "couldn't they rerun some season two (if not season one) episodes?", especially since I don't think it was in reruns yet.