DIY potting mix problems

I made my own potting mix up this year and last year, using a mix of well rotted compost, well rotted wood chip mulch, biochar and fine granite rockdust for trace minerals. The plants grow strongly, but the leaves yellow and die in many spots.

My research suggests that the nitrogen levels are fine, but that I have a serious magnesium deficiency in my mix. Any suggestions for fixing this in future years? I have been using a weekly epsom salt foliar spray, but seeing no noticeable improvements. Main problems seem to be with strawberries in a pair of large planters.

I understand your frustration.  My mixes have inconsistent results.  I think it is because the soil life has not stabilized the availability.  Older mixes that have gone through a cycle of roots seem to do better.
Strawberries are very sensitive to  the crown being at the right position in the soil.   I had to transplant a tomato again because the new material I had added to the container was not supporting new root growth.