I would plant the Photinia in John Innes No 3 loam based compost - it will contain all the nutrients needed for the first 6 months. After that I would top dress in springtime with a sprinkling of Fish, Blood and Bone.
Tomorite is formulated to encourage flowering and fruiting which is not what you want the photinia to do - it's glory is in its new leafy shoots so needs a more general-type fertiliser and FBB is slow acting and will fertilise the whole plant, not just it's fruiting potential.
Loam-based compost is also heavier than soil-less composts and will help prevent the shrub being blown over in the winds.
Make sure the contaner has plenty of drainage holes, and you put plenty of crocks in the base.. However, metal containers transmit heat/cold very easily - I would wrap the outside of the pot with bubble wrap in the winter, to protect the roots from frost, particularly in the first winter. Similarly be aware that in hot weather the container may heat up and this may scorch the roots.
I would prefer to use a different type of container for a long-term planting - or plant the shrub in a pot which can then fit inside your zinc container with some insulation between.
Hope that's helpful