I bought a very bushy ficus burgundy (rubber plant) in a 10' pot about 6 weeks ago. It has many stems with many leaves all over (you could not see the soil at all without some serious parting of the leaves, and many leaves were probably not getting any light on the inside). With time in my apartment lower leaves started dropping a bit - which I understand it is partially a normal process of going from ideal nursery environment where I heard they treat these plants with hormones to encourage bushiness to my relatively low light environment.

What worries me is that some leaves get these unhealthy looking spots on them before dropping. The spots are brown to tan colored, the leaves thin in those spots, the veins become more pronounced to the touch and sight, the spots feel like very fine dry sand paper rather than the usual thick smooth leathery feel of a healthy leaf. Is this normal for leaves about to be dropped, or is is a bug or some sort, or perhaps it's in the care I provide?

The plant is in the original nursery soil - I have not repotted it. The mix seems rather fluffy with some bark, what looks like peat and controlled release fertilizer added in. I have not seen the soil compact too much between waterings, which is one of the reasons I have not repotted.

The plant is in front of a west-facing window with no direct sunlight because of tall buildings and trees around.

I water when the soil becomes dry, then soak and dump the saucer water. In practice that meant:
5/25: first watering after bringing it home. I also gave it a good shower because the leaves had whitish residue on them, perhaps fertilizer, pesticide or hard water marks. I wiped the leaves with a soft cloth trying to get rid of the residue, too.
6/14 (20 days later): another shower and leaf wipe. I also fertilized at that point.
Today (18 days later) the soil is still a tiny bit damp, so I did not water.

Fertilizer (only once so far): weak Foliage-Pro solution (quarter teaspoon per gallon) - and that water also had vinegar added in, as my water is alkaline.

Any advice to prevent these spots and to minimize leaf drop?