You need to reassign input (or use an intermediate variable). What's happening in each loop is you are taking the fresh input and drawing the text over that.

You could do something like

Copy...
        input = input.drawtext(
...

I believe, though I haven't worked with videos much. Like, you might need to do more to make sure the text doesn't all just layer over itself.

A colon ":" and a backslash "\" have special meaning when specifying the parameters for drawtext. So what you can do is to escape them by converting ":" to "\:" and "\" to "\\". Also you can enclose the path to your font file in single quotes incase the path contains spaces.

So you will have

ffmpeg -i C:\Test\rec\vid_1321909320.avi -vf drawtext=fontfile='C\:\\Windows\\Fonts\\arial.ttf':text=test vid_1321909320.flv

This is your basic template. I assume the input is 720p.

ffmpeg -i 720p_input -f lavfi -i "color=black@0:s=64x64,format=yuva420p" -filter_complex "[1]trim=end_frame=1,drawtext=font={font_family}:text={watermark_text}:fontcolor={font_color}:fontsize={font_size}{background_box}:{watermark_position},rotate=a={degree}*PI/180:ow=hypot(iw,ih):oh=ow:c=black@0,loop=-1:1:0,tile=15x9,trim=end_frame=1[wm];[0][wm]overlay=0:0" -c:a copy -preset encoding_preset ./watermark/{media_id}.mp4

{degree} should be replaced by rotation angle. The ultimate size of each text cell is 64 x sqrt(2) = 90 pixels on each side. 15x9 is the tiling layout. Adjust this to cover the full input frame.

Positions

• Top left: x=0:y=0 (with 10 pixel padding x=10:y=10)
• Top center: x=(w-text_w)/2:y=0 (with 10 px padding x=(w-text_w)/2:y=10)
• Top right: x=w-tw:y=0 (with 10 px padding: x=w-tw-10:y=10)
• Centered: x=(w-text_w)/2:y=(h-text_h)/2
• Bottom left: x=0:y=h-th (with 10 px padding: x=10:y=h-th-10)
• Bottom center: x=(w-text_w)/2:y=h-th (with 10 px padding: x=(w-text_w)/2:y=h-th-10)
• Bottom right: x=w-tw:y=h-th (with 10 px padding: x=w-tw-10:y=h-th-10)

Full example (centered)

ffmpeg -i input.mp4 -vf "drawtext=text='Super User':x=(w-text_w)/2:y=(h-text_h)/2:fontsize=24:fontcolor=white" -c:a copy output.mp4

See drawtext filter documentation for more info.

Just chain the drawtext, at the end.

ffmpeg \
-i video1.mp4 -i video2.mp4
-filter_complex "[0:v:0] [0:a:0] [0:v:1] [0:a:1] concat=n=2:v=1:a=1 [v][a];
[v]drawtext=text='SOME TEXT':x=(w-text_w):y=(h-text_h):fontfile=OpenSans.ttf:fontsize=30:fontcolor=white[v]" \
-map "[v]" -map "[a]" -deinterlace \
-vcodec libx264 -pix_fmt yuv420p -preset $QUAL -r $FPS -g $(($FPS * 2)) -b:v $VBR \
-acodec libmp3lame -ar 44100 -threads 6 -qscale 3 -b:a 712000 -bufsize 512k \
-f flv "$YOUTUBE_URL/$KEY"