When I see the many queries in this forum on starter quality there is often an emphasis on judging a "ready to bake" starter by its by bubble-activity.
The starter that I use is about two years old, and is refrigerated most of the time. Each week, I mix 60g of "old" starter" with 60g of water and 60g of flour (equal parts of APF and WWF). When I'm ready to bake I let the starter warm to room temperature and I feed it once or twice until fully ripe. This means (to me) that my starter has more than doubled (within eight hours). When ready to bake, I see little bubble activity.
The result of my bakes are nearly always as expected - good rise, good crumb, good taste.
So, should I be concerned about the lack of bubble activity in my starter? Does this indicate a starter not as potent as it should be? How would a more active (bubbly) starter improve my bread?
If your starter has risen to more than double, it has bubbles in it. The bubbles are what cause the rise. Since you are using 50% WW flour, your starter might be thick enough that the bubbles can't rise to the surface quickly. So maybe you just aren't seeing them? Is your starter in a clear container so you can see bubbles from the sides or bottom? Perhaps you have a large number of very small bubbles so you are discounting them? Another possibility is that the bubbles are rising to the surface, and popping, which would also explain why you don't see very many. But that would also result in less rise so I don't think that's the case with your starter.
As far as what a more bubbly/active starter would get you, I have to go into territory where I'm making educated guesses. Hopefully if anything is inaccurate, someone else will speak up. I can say for sure there is a finite amount of C02 (bubbles) that can come from a certain amount of your flour. So to me, it seems like more active starters don't really do "more", they just do it faster. If you were able to tweak your starter to be more active, you might be able to cut down some of the resting times in your recipe, because your little pet yeasts and bacteria are doing their job faster. My understanding is that this is not necessarily a good thing because longer times lead to more flavor. But if you desire a more mild flavor in your bread, you might want this. However, it is also my understanding that you can also speed up the times by tweaking your recipe slightly to include a higher ratio of starter and reducing water and flour to compensate. So you can accomplish at least nearly the same results even with a less active starter.
If you are happy with the results you are getting, and you've been getting those results consistently for 2 years, I would not worry about the health of your starter. Just keep doing what works for you.