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I see a pest invasion, looks like scale or possibly spider mites, but I don't see any webs. If you have some Neem oil, spray the leaves with it, following the directions on the container. Insecticidal soap will help, if you don't have Neem oil.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/pest-control-for-plumerias.htm

Plumerias should be protected when ambient temperatures are approaching the 40°F degree mark or so. They can be moved to any location where temperatures will remain warmer than that. However, the shorter days will result in less chlorophyll production and foliage will start to fall unless steps are taken. Plumerias will usually grow and flower under fluorescent tubes or energy saving bulbs, as well as under LED lamps.

You can try cutting the stalk in half - and rooting the top portion. The bottom stalk should resprout with multiple branches, however, there are no guarantees. Also, when it blooms, branching should occur.

Is it getting full sun? If so, it may bloom soon.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria-cuttings.htm

I can sympathize. I, too, have gone through all the trials and tribulations of growing Plumaria. Does yours ever get outside? It could be outside most of the year there. If you can add more perlite, I think it would help. Work it into the top without disturbing the roots. You can add it to the top if you can't work it in. Then it will work its way down over time. The soil needs to be very well draining, then if you overwater, it will just run out.

Next spring, I suggest cutting off about 12 inches and rooting that. Then the bottom part should branch out. Do you read or belong to any of the Plumeria societies? They have good care information. So do the plumeria groups on Facebook.

https://southerncaliforniaplumeriasociety.com/

https://southerncaliforniaplumeriasociety.com/care/basic-care/rooting-cuttings/pot-rooting/

https://www.gardeningknowhow.com/ornamental/flowers/plumeria

This appears to be a very small and slightly malformed leaflet. It is possible that it is lacking some nutrients. This can be tested and corrected, however. This would explain the failure to bloom, as well.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-does-not-bloom.htm

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/how-to-fertilize-plumeria.htm

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria-plants-indoors.htm

The green pods will take about ¾ of a year or more to mature while still attached to the tree. As they approach maturity, the pods may color change and turn brown/black and also may shrivel/dry out. Once mature, the pods will open down one side. You can collect the seeds using cheesecloth or a panty hose leg. If the pods fall down prematurely, they may not be mature enough although some people still try to hang them to dry in sunny conditions though I am not sure if that is ever successful (or what percentage of the seeds becomes viable). In those cases, you may need to see if the seeds are viable: some people place the seeds in water for a fewl hours to see if any viable ones sink. Viable ones should have a plant embryo (the thick end of the seed opposite the papery end) that is starting to swell. Then, soak dry seeds overnight. Put the seeds in a well-drained, sterile potting mix, embryo side down and barely cover the seeds. Wait about two weeks(?) for the seeds to germinate. You can also go to Facebook, search for the DFW Plumeria lovers Group and join the group.