Audio and Video out of sync when slowing down
The reciprocal of 75% i.e. 0.75 is 4/3 or 1.3333, so

ffmpeg -i combinedfullspeed.mp4 -filter_complex "[0:v]setpts=(4/3)*PTS[v];[0:a]atempo=0.75[a]" -map "[v]" -map "[a]" slow3.mp4