The animal is thriving and the plants keep dissolving, how is the lighting wrong when the tank looks bright?

The practical answer is usually two fixtures doing two jobs, and there is no shame in it.

The light that came with a terrarium kit is generally sized to make the tank viewable and to provide whatever the animal needs. That is a legitimate job and it is not the same job as growing plants at the bottom of a tall enclosure.

What to look for in a plant light for this:

A fixture that states output for growing, rather than one sold on wattage or brightness alone. The specification you want is about photon output in the useful range.

Enough to reach the substrate. In a tall tank, that means a stronger fixture than the same tank would need if it were short, and it is worth choosing on that basis rather than on tank volume.

A sensible photoperiod, in the region of ten to twelve hours on a timer. Longer does not compensate for too little intensity, plants need a threshold, not a total, and extending a dim day mostly just grows algae.

And check the animal's requirements are still met and not exceeded. A brighter tank changes basking behaviour and the amount of cover a shade-dwelling species wants. More plant cover, which you are about to get, usually helps with that: the two things solve each other once the plants actually grow.

Light is not light, and the mismatch is the most common reason bioactive planting fails while everything else goes well.

Your eyes are the problem. Human vision adapts over an enormous range and is most sensitive to the middle of the spectrum. A room that reads as bright to you can be a small fraction of what a plant needs, and you cannot perceive the difference because your eyes have quietly adjusted. Everybody says the tank looks bright. It is not a useful measurement.

Plants respond to photon count in the range they can use, which is a different quantity from apparent brightness. A fixture sold on lumens is optimised for looking bright to you, not for driving photosynthesis.

Distance punishes you harder than you expect. Light falls off sharply with distance, so a fixture that is adequate at the top of a tank can be well below the useful threshold at the substrate. In a twenty-four inch tall tank that difference is large, and it explains your specific symptom precisely.

Leggy then rotting at the base is the classic under-lit progression. The plant stretches toward the light, spends its reserves doing it, thins out, and then the constantly damp base of a weakened plant rots. It is not a rot problem with a lighting coincidence; the rot is the last step of the lighting problem.

Two things you can do without buying a meter.

Use a phone light meter app as a rough guide. It measures the wrong quantity, so do not treat the number as authoritative - but it is excellent for comparisons. Measure at the top of the tank and at the substrate and look at the ratio. In a tall tank people are routinely shocked by how much is lost. That comparison alone tells you whether your problem is the fixture or the distance.

Pick plants for the light you actually have. This is the cheaper direction and it works. If you cannot get strong light to the bottom, plant the bottom with things that tolerate low light and put the demanding ones high - on the background, on branches, near the top. Layering by light requirement is what makes tall vivariums work, and fighting it by trying to grow a sun-lover at the substrate is a losing proposition regardless of fixture.

Things that generally cope with less: many ferns, pothos, some philodendron, mosses if the humidity is right. Things that generally do not, and are commonly sold for these tanks anyway: most bromeliads, anything with strong red or variegated colour, and most succulents.

Given three failed rounds, I would do both - better light and a more forgiving plant list - rather than betting the fourth attempt on one change.