PTS: Format to S:MSS

Question

I'm using ffmpeg in a bash script to have a timer overlay in the video:

fps=30
seconds=5.53;
lowerFont=30;
fpath="./input.mp4";
ffmpeg -y -i "$fpath" \
 -c:v libx264 -r $fps -t $seconds \
 -pix_fmt yuv420p \
 -vf "fps=$fps,\
 drawtext=fontfile='/mnt/c/WINDOWS/Fonts/HyliaSerifBeta-Regular.otf':fontcolor=yellow:fontsize=$lowerFont:\
 x=(w-text_w)/2:y=((h-text_h)/2)+$lowerFont:text='%{pts\:gmtime\:0\:%S\\\\\:%MS}'" \
 "timer.mp4";

(excuse the messiness)
This shows M:S well but I need it just to show S:MS and gmtime doesn't show that. I've tried using hms and flt but I can't seem to format that in any way. How can I achieve this