I'm looking for synonymous and antonymous expressions of "go easy on spices" and "go overboard with spices." Can you think of others?
I'd suggest simply don't over-season, versus ... and don't skimp on the seasoning. There are so many synonyms for “don't overdo ...” and “be generous with ...” that it's impossible to list them all (now someone will try).
For salt and sugar, “go easy on” is probably just about the most universally understood expression to use. No need to try too hard :o)
For practical examples, I'd suggest perhaps checking out some TV chefs / recipe books for what's considered normal (by them). Just don't treat Lloyd Grossman as an authority, he talks very oddly. Being British, I'm thinking of Nigella, Delia Smith, Anthony Worrall Thompson, Ainsley Harriott, ...