Add film grain, resize and create SBS file for stereo 3D

I have 2 files at 1080p resolution.

I wish to resize each to 1920x2160, combine both into SBS 3840x2160, add subtle film grain, add burn subtitles into each eye and finally create an SBS 4K stereo 3D video file with CRF 18.

I have used ffmpeg previously but this is way beyond my understanding for the filters.

Help would be appreciated.

By 1080p, I assume you mean 1920x1080. Use

ffmpeg -i left.mp4 -i right.mp4 -filter_complex
   "[0]noise,subtitles=subs.srt,scale=1920x2160,setsar=1[l];
    [1]noise,subtitles=subs.srt,scale=1920x2160,setsar=1[r];
    [l][r]hstack" -crf 18 -c:a copy -metadata:s:v stereo_mode=left_right out.mkv

See the docs for the noise filter to alter its default values.