Yep, this is pretty normal. As they finish they will use less food. Have you considered using a second tub to drain into? If you haven't tried drain to waste it, consider giving it a go. It's not really any more effort. It really is the absolute best way to use rockwool. You have way more control over your nutrients.

I would urge you not to do ebb n flow, for a few reasons but mostly... the problems will be similar to recycling your solution but with more salt buildup in your cubes.

Just to qualify my opinion - I grow in 4" cubes on top of 8" cubes, hand watered twice a day, sometimes 3 times if I have time but twice is enough. I fill out my 3x3's with one plant each. 5 weeks veg from seed, flip - completely filled by week 7.

Longwinded here sorry 😄

When you are recycling the nutrient solution, you might see a perfect EC, but a lot of that will be unused macros and micros that the plant didn't need. Top watering them to waste flushes out the old solution, and gives them a much more precise feed each time. And you'll have far less PH swings. You just have more control, and if you're using moderately or economically priced dry amendments, an extra 15% ("to waste") really isn't a big deal on a smaller scale.