Footleg
This was one of my early challenges shortly after discovering Therion, and it crops up from time to time on this forum.
I have decided that with ‘draw once, use many times’ I am limited to about half an order of magnitude difference in scales (ie 1:500 to 1:2000) to get label spacing and symbol placement that is more or less OK at a range of scales.
Personally, once I get to 1:5000 I pretty much turn off all symbols. http://therion.speleo.sk/wiki/doku.php/templates
Judicious use of text scales xs, s, m, l, xl allows one to crudely hide minor text at smaller scales.
You can use base-scale to globally reduce all text and symbol sizes.
Thomas Holders examples show how symbols can be made to plot to suit the scale. Ie hide smaller symbols. http://therion.speleo.sk/wiki/doku.php/metapost?s[]=thomas#scale_dependant_visualization_of_symbols
People have proposed various ‘dual scrap’ scenarios, but it seems to defeat what simplicity Therion has :)
You could use ‘revise’ for each symbol, but I think madness would lie down that route.
Check the wiki.
Bruce