ffmpeg: can I crop using vf while also using filter_complex at the same time?
Use this:

ffmpeg -i input.mp4  -filter_complex \
"[0:v]crop='if(gte(iw,ih),ih,iw):if(gte(ih,iw),iw,ih)',scale=720x720,split=3[1v][2v][3v]; \
 [1v]trim=0:4.95,setpts=PTS-STARTPTS[v1]; \
 [2v]trim=4.95:6.75,setpts=(PTS-STARTPTS)/0.1[v2]; \
 [3v]trim=6.75:8,setpts=PTS-STARTPTS[v3]; \
 [v1][v2][v3]concat=n=3:v=1:a=0[out]" \
-map [out] -an -c:v libx264 -profile:v high -level 4.1 \
-preset superfast -crf 20 -r 30 output.mp4