Should I start lightly nuted, pH'd water now or wait? Why? What clues tell you the autoflower is ready?

First of all check your water ph. Soil buffers slightly yes, but it cannot if the water is too extreme from one scale to the other. Eg. -5ph or 7.5ph+. She wants nutes badly, you can tell as all her leaves are yellowing, add some grow nutes as soon as possible at 1/2 recommended strength. 🌱👍

yellow underneath.. go for it.. in future judge by teh little cotyledon leaves.. they start turning yellow, that means the plant is cannibalizing itself.. time to feed. your first serrated blades are yellowing from tip in.. that's likely nitro-def, this early fairly safe to say.

Does your soil have any added nutrients in it? or is it an inert soil with no nutrients added? Soil like Fox Farm Ocean forest with added nutrients can last 3-4 weeks before any feeding is needed. If not thou i'd start at a 1/3 str dose of nutes every other watering now. The lighter color of your plant makes me think it's begging for nutrients already so i'd say start next watering like your thinking! Happy growing! 👊