The regular pattern of yellow is unlikely to be fungal or bacterial which is almost always random.
Interveinal chlorosis (yellow between the veins) can indicate deficiency of iron, manganese or magnesium.
Iron is NOT mobile in the plant so it appears on young leaves first and older leaves not affected.
Mg and Mn are both mobile so when there's not enough coming from the roots, the plants rob from old leaves to keep the new leaves healthy. Symptoms appear in older leaves and not much in the newer leaves.
Please check whether the pattern is in old or younger leaves or all leaves.
Iron deficiency used to be common in soilless potting media but modern fertilisers usually have extra trace elements to counteract. Modern, inorganic bonsai soils are also prone to nutrient deficiency as most can't hold much without regular applications. Alkaline water can make it harder for plants to access Fe even if it is in the soil.
If you are waiting for osmocote to disappear that's probably way too long between fert. The skins are still there way after all the nutrients are used up. Need to check what release time your osmocote is and apply more at those intervals. Also not sure what your definition of 'moderate amount' is but I'm guessing not enough for ficus which are usually hungry plants.
I'd try a liquid fert first. Most will have all the trace elements, including Fe.
Iron chelates is another option. It adds extra iron and acidifies the soil to make it more available. Can be a quick, short term fix for Fe deficiency but you'll need to address the issue longer term to prevent it recurring.
. I do have fish emulsion that I use for my trees outside. I'll try the iron chelate and then the fish emulsion. It seems like it's due for a growth explosion soon, too. So I should find out quickly how things work out.
