What haven't I tried to get a sour taste

If you want to increase the sour taste of the bread, try retarding the dough.  Make your dough, then place it in the refrigerator for a day or two.  Form your loaf and bake it.  If that is not sour enough for your taste, then increase the time of retardation and also retard the final loaf.

I once made some refrigerator rolls from sourdough starter.  After about a week of the unbaked dough being in the refrigerator, the baked rolls were much too sour for my taste.

Ford

50% hydration for a starter is extremely low.
100% is what I use. Some here even use 50:50 in volume. This is about 170% hydration.
The MOs will get more active in wetter condition even tough you might not see a rise as the dough might not keep the bubbles.

Another idea: Do you put any spelt/wheat in your starter. Just 100 rye. If you didn't have at least a high percentage of rye, it might take a few weeks to develop the sour aroma. Better to start a new one then.

Another idea: refreshing mainly increases the amount/activity of yeast compared to the bacteria. It is the bacteria that produce the sourness.

Use more sourdough (starter) in your final dough.

Liebe Grüße
Adrina

I have been trying to get a much sour out of my starter as I can and have, like you tried all kinds of ways to get there. I have found what works for me.  I retard the starter, at 36 F for at least 7 days before using  it to make bread.  It is fed whole home ground rye only and is developed at 92 F on a heating pad on the counter - a real proofing box would be better.

For a real sour bread I will use 15 g of the starter to make a levain that is 20% of the total flour and water weight of the dough.  I use a 3 stage levain build, all at 92F.  The first build is twice the weight of the flour in the starter and my stored starter is at 66% so it has 9 g of rye in it and 6 g of water.  The first feeding is 18 g each of rye and water added to the 15 g of rye sour starter.   This sits on the heating pad at 92 F for 2 hours.   They the 2nd feeding is twice the first one so it is 36 g each of rye and water, don't throw anything away and it sits on the heating pad until it doubles about 4 hours or less.  You now have 123 g of levain. 

The 3rd feeding shoots for what ever you are making hydration wise and how much dough you are making. If you are making a 1,000 g loaf at 80% hydration then you multiply 1,000 (total dough weight) by the 20% you want to be levain which equals 200 g and i make the last feeding of the levain equal the .hydration of the bread.  so the 200 g of final levain divided by 1.80 (hydration fo 80%) gives you 111 g of flour ....and 89 g of water. 

You levain build after the 2nd feeding had 9 g of flour +18 +36 = 63 g of flour and 6+ 18+36 = 60 g of water.  So the  final feeding is 111- 63 = 48 g of whole rye and  29 g of water to make 200 g total at 80% hydration.  This goes on the heating pad at 92F until it rises 25% in volume than it goes into the fridge at 36 F for 24 hours.  Then it comes out and is allowed to finish its final doubling on the heating pad at 92 F.  Them it is ready to use.

Make sure to do all your work as much as possible for gluten development at 92 F as much as you can - no matter how you develop the gluten -  mixer with dough hook, .slap ad folds and or stretch and folds.  Then shape and put in baskets or molds or what ever and immediately refrigerate  the shaped and molded dough at 36 F until it proofs to 85% 12 -24 hours.  Start the oven and when the stones are at temperature, remove the dough from the fridge, slash it and bake it straight out of the fridge with mega steam.

This will give you the most sour bread your starter can manage, except for another way that is slightly more difficult that required a bulk retard shaping and final proofing at 92 F to 85% proof before baking,

The things to remember are tiofeed the starter and levain wholerye fresh ground flour or as close as you can get to it.  Retard the starter, levain and dough at 36 F as ;long as you can.  Do everything at either 36 F or 92 F and as little at room temperature as you can.

To get even more sour - increase the levain from 20% to 30 to 40 ....even 50%..

This is what makes sour bread for me. 

I've read that for the most sour, rye is the way to go. But I haven't tried rye, so I don't know if that's true or not. What I have done is change to feeding whole wheat flour to my starter. I started by adding a little to my white flour starter, and increased at each feeding until I was feeding it only whole wheat flour. I was keeping it on top of the fridge and feeding twice a day. When I made my bread from this, I used about 20-25% starter in the bread, along with just plain white bread flour, water, and salt. The bread was super sour. I liked it, but nobody else did, so I switched back to feeding just white flour to my starter since then. I often let my dough bulk ferment for several days to a week in the fridge, and it gets quite flavorful that way, but never nearly as sour as it did feeding WW flour to it. A combination of the two would probably yield even more great flavor, and intense sour. Using rye-fed starter in combination with a long retard may be the extreme end of it, if what I've read is true.

SDN - I literally went years, thinking my particular location was just too hostile to the right bacteria. I kept white motherdough at 70% hyd. and white & wheat at 100% hyd. They were kept in the fridge. 'Rarely refreshed more than once before mixing.
When my white starter looked like it died, I started a new whole wheat starter, and commenced refreshing the white, in hopes that I wouldn't have to do w/out baking for the 2 wks it would take for a new starter to gain strength. In 2 wks, my wheat starter was strong, but the white was the big deal - not only did it raise dough well, but it came back w/ a strong tang & strong odor of vinegar. Bread was delicious (I like tang).
Since then, reading Debra Wink's threads here, I see that warmer temps w/ full hydration seem to favor the acetic acid (tang). I've been keeping my starters in the garage in a cabinet where it stays about 43-48 deg. F. The tang is holding. If it falls off, I'll give it a number of days at normal room temp, refreshing at least once/day. I don't enjoy having to refresh daily, but the tang has been worth it for me.
'Wish you the best of luck w/ your efforts.