Please help diagnose my friend: my 10 year old orange tree :(

I have tried finding what's wrong with my 10 year old, very well established but now dying orange tree. Plenty of fruit finally ripened at the start of summer (here in New Zealand) and you could see that the leaves were very weak and looked old. We started feeding it compost as well as magnesium (which is deficient in New Zealand soil) (but no citrus specific fertilizer). However, a few weeks later, leaves started dropping by the hundreds daily! So I would doubt it is nutrients. Now it has around 20 very small, young leaves but then just the last 2 or so days these tiny leaves which look otherwise healthy also started dying, as if they were sprayed with insecticide/dried out from draught (i.e. yellowing and curling up), however it has been quite rainy in New Zealand!I have looked for signs of every common disease: sooty mold, aphids, etc. Whilst the older leaves do have persistent characteristics with a few of these parasitic diseases (e..g a few have reminants of insects and black soot in very limited amounts), i don't think that they would lead to my poor tree suddenly dropping 90% of its leaves in just a handful of days. What I have noticed in most leaves are very tiny black dots. I.e. too tiny to be sooty mold (<0.5mm), even in the very young but now dying leaves (however in very tiny amounts and sizes [<0.1mm], not warranting their deaths!). These are inbedded in the leaves and cannot be scratched off. However, the few leaves yellowing of the perfectly healthy lemon tree sitting right besides my poor orange tree show exact;y the same signs as the older leaves of my orange tree.I have looked at the bark and although some branches have began drying out, it is still mainly green, I hope for long enough for it to heal itself! I don't think that any particular disease (or the combination of diseases it may have in managable mounts) warrant the sudden drop of leaves. This is specially so for the new leaves, so tiny that would have no chance to be infected just yet! SO I WONDER WHY MY POOR TREE IS SO STRESSED! :((( I am so sad, to think that I've keep this tree for a decade and it decides to die so young and suddenly. Could it be that the tree remains stressed from the nutrient defficiency months ago? The thing is that it is not in the path to recovery. The young leaves were in the process of growing as the fruit ripened and then just suddenly decide to die a few days ago after the older ones decided to drop?????As mentioned, the leaves were weak for months, during the fruiting stages. Although we started feeding, this could still be a possibility, but why would the leaves drop so quickly and even the young ones? Surely the nutrients would have entered, in the last month or so of feeding. The climate has been very rainy, humind and warn but it has good drainage and then, again, the leaves were quite weak from beforehand.If anyone has any possible idea of how to help my orange tree increase its survival chances, my 🍊 tree will thank you for its next few decades! I would love to consult an orange tree expert but I am very sure that they don't exist. Garden centres will probably just sell me fertilizer and spray for black soot even though I wouldn't think that these were issues. Why do all the signs of what could be wrong make no sense and contradict each other? 😭​If you can't figure out what could be wrong (and I definitely don't blame you) how long do orange tree survives with no green leaves whatsoever? 😭😭 I am so sad.