What config should I use to improve the zoom texture quality?

improve resolution, like if it's vlowres now, use lowres, is lowres, then highres, or your own perspectives where you define scale

if you don't see your world in worlds.txt, then execute dmap worldset worldname title:<label>, where label is a custom name for the world, like "Overworld", then your world will appear in worlds.txt
after that, search for all perspective: <name>_<resolution> and change resolution to a better one, and save worlds.txt
and finally, restart the server, purge rendered data, and redo a fullrender for the world
resolutions are: (resolution-explanation, higher scale= better quality in zoom, more lags from render)
vlowres - very low resolution (scale:2)
lowres - low resolution (scale:4)
medres - medium resolution (scale:8)
highres -high resolution (scale:16)