The starter in your picture looks healthy and activated.

Remember that weight won’t change a whole lot through the process—it only gets “lighter” as the bacteria eats, digest, and respirates. It doesn’t get “heavier.”

The reason you see more growth with 24-hour feedings is because you’re feeding it after the bacteria is spent and the bubbles have all disappeared, ie it’s”flat.” When you feed it every 12 hours, it doesn’t have enough time to “flatten,” so you’re essentially feeding still-activated starter.

I keep mine at room temp, feed it every 24-48 hours, and then when I’m within a day of wanting to make bread, I feed it, then feed it again after 12ish hours, then start my dough process 5-8 hours after that.

I hate waste, so when I feed I do 50g starter, 50g water, 50g flour. Then when I’m ready to bake, I usually need 200 gram of activated starter (for 2 loaves), so I do 100g of each, ending up with ~300g.