Interesting idea, I will try to explore it. There is also linux command to add watermark to picture, I will check if that can be used in script shell. It may be needed to use Termux to install something and then run termux command from within script shell...
It can be done with termux and imagemagick installed by command like
convert -background white -fill black -colorspace gray -depth 8 -font Roboto-Regular -size 360x720 -pointsize 20 -gravity northwest -border 10 caption:"This is some text as image" 1661239525792.bmp
However, now I need to find out how to run termux script without termux:tasker plugin. I have old Android 6 and termux:tasker does not install