Many leaves have dark coloured dead patches, whole leaves discoloured and dying, no obvious pests, diseases or soil issues.

You've loved your plant to death!  Well not quite.  Many houseplants are placed in decorative cache pots and unfortunately one of the main reasons they die is that they are watered too much.  Another cause could be too much fertiliser.

Remove most of the affected leaves, leaving at least one. You can also trim leaves that have only a small patch. Repot into a well draining potting mix, removing as much of the existing soil as possible and rinsing roots thoroughly. Don't water for a couple of days and then water weekly or when leaves start to droop.

Consider giving your plant a holiday and taking it out to a verandah or well shaded spot in the garden.  When bringing back inside, look for a different spot that has a bit more light and air to try and avoid a repeat situation.

Leaves curled and discoloured, black insects present inside folded leaves, new leaves targeted.

You've got Thrips!  Ficus benjamina are particularly prone to Thrips and if your tree is outside, there is little you can do to eradicate them all together.

For smaller plants, remove all affected leaves then apply a pyrethrum spray such as Yates Pyrethrum concentrate.  Mixed with water, apply over all leaf, stem and soil surfaces and repeat at three days then weekly until Thrips are no longer apparent.  Continue to monitor each week for a month.

As the tree gets larger, application of spray treatments are very difficult so consider using a granular insecticide such as Richgro Bug Killa.