Why might the main branch of my plant be too thin?

no air movement to strenght up stem, maybe watering issues, root system weak = plant weak give bigger pot and light breeze for her.

To much growth upwards mate. Stretching. Use a phototone app.. 15-20k lux you're far off by the looks of it. You can fix this, when u uppot it once the stem is green. Plant in lower in the pot and fill in to higher your sub to meet level needed.

Your plant needs more light that's why she is stretching in search of light, lower the light or increase the power

Wicket, gave the answer. Not enough light.

take a note* (originally 'not')

First two internodes are very long. The top one might be okay? If you recently increased light intensity, it might be fine, otherwise it is about to stretch like the first 2. More light if you haven't already done so. If you have, keep an eye on that top-most internode. Don't let it stretch like the previous two, but amp up incrementally and see how it reacts before doing more. take not of power and distance. Use that combo next time without the trial and error.

Light too weak or too far away. Replant it as deep as you can when you put it in the next pot.