Yes sow spinach now, and for next winter remember to sow it in early September. We were just picking some lovely leaves in the tunnel. Whereas sowing now, you won’t be picking before April.
Re your compost question, I don’t think you need to be so precise about mineral contents, esp for compost which is as much about life organisms, rather than being a precise supply of nutrients.
There is such advice out there, but I am not convinced how much chemists understand the biology of a compost heap, and I don’t claim to understand either but know that I can make compost that grows fine plants, by observing.