Starter triples in four hours but my loaves still bake flat and dense Troubleshooting

Your starter is fine and your bulk is almost certainly too long. Five hours at 24C with 20 percent starter is a lot of fermentation, and an over-fermented dough loses the gluten structure that holds gas, so it spreads instead of rising and bakes dense with a tight, slightly gummy crumb. Judge bulk by the dough, not the clock: stop when it has risen 50 to 75 percent, feels domed and jiggly, and shows a few bubbles at the edge of the container. In your setup I would expect that at more like three and a half hours. Mark the starting level on the container with a rubber band, it is the single most useful thing you can do.

Try one loaf with 50g starter instead of 100 and everything else identical. Halving the inoculation doubles your margin for error on timing, and the flavour is better too.

Update: bulk cut to three hours 20 minutes, ended at about 60 percent rise, same everything else. Loaf came out 9cm tall with an open crumb. It was the clock.

Second the over-fermentation, and add one thing: the float test tells you a starter has gas, not that it is at peak. Tripling in four hours means you should be using it well before four hours, at or just after peak. A starter that has already begun to fall is more acidic and degrades gluten faster in the dough.

Shaping is the other candidate. A dense flat loaf with a strong starter is often a dough that was never given surface tension. After preshape, rest 20 to 30 minutes, then shape firmly enough that the skin is tight and the ball holds its shape on the bench for a minute without slumping. If it slumps immediately you either over-fermented or under-shaped, and those two are the whole list.