Hi Kags
Never too late! Soil organisms are always happy to be fed. Just that mulches sooner before sowing need to be finer, so if your ‘manure’ is in fact compost, i.e. crumbly and decomposed, you can spread it now or even later.
If not, I would buy and spread some finer compost, even if only a little. The rockdust is a good idea, asap, again to feed soil life. Though it’s not a compost substitute, but an extra.
