I have a rubber plant that has grown sideways and quite tall. I want the plant to become somewhat dense and a little less tall. I am thinking of pruning it as shown in the image below:
However, I am not sure if it is okay to prune so much of the rubber plant and then what to do with those bigger stems (propagate? Green or brown stems?). Or should I let this plant get much bigger and grow freely?
Rubber plants (Ficus elastica) can be hard pruned and can respond well. In the wild they are trees, so they can certainly get bigger if you want. Just bear in mind that that pot is not very big for the size of plant, so pruning may buy you time before you need to repot.
Cuts 1 and 2 look about right - it will look better if you can just above a branch and certainly always cut just above a node. Cut lengths of stem above a node will die back to a node. Cut 3 doesn't look to be a particularly sensitive choice - personally, I would go a little higher and cut just above the branch that is growing up away from the marked position.
Up to you what you do with the prunings! There is certainly material you can use to propagate it. You need a length of stem around 100mm long. Remove the lower leaves, leaving just the growing point and one expanded leaf - they don't need to photosynthesis much at this stage and will dry out if there is too much leaf area. Insert into a free draining medium - potting compost with added grit would be good. Try to ensure that at least one node is below the surface of your compost as this is where the roots will form. Do not overwater, as they will rot easily, and until they root, won't be taking up very much water. Experiment with different types of material - green or brown stems. The first time I propagate a plant, I try a range of materials to see what works for me. Ficus species root very easily, so you should get some new plants.
A closely related Ficus Benjamina I had many years ago, had the same problem: Bottom leaves falling off.
One thing which stood-out when I attempted to repot it, is being rootbound. After a while, the roots are too big to keep spreading, and will start turning around the edges of the pot. Given that (from your description) it did not start immediately to turn yellow, assuming it gets all the essential nutrients, root condition comes to mind. Ficus trees grow rapidly. So do their roots. You may take care of it just the same way you care for a Bonsai: Trim the branches, but also don't forget the roots. If they wrapped around, you must untangle them and trim them back to where they started turning around. No need to trim further and there's no need to make it look like a bonsai, but trimming the roots is essential for potting-up if the roots are bound and tangled.
