Increasing starter strength

It's best to keep your starter relatively cool, below 77 degrees if you can, because yeast functions better in an acidic environment at lower temperatures.  When you divide and feed your starter be sure to aerate it with a whisk or similar tool to incorporate oxygen so the yeast can work more easily.  Without sufficient oxygen the yeast can't build cells .  Dividing the starter when feeding does two things.  The newly added water dilutes the acids which tend to interfere with the starter's growth and provides an improved food supply for the hungry yeast. Try feeding it twice a day for a couple of days, oxygenting it each time.

"Perhaps this flour is lower in protein"  The nutritionallabel on the side of the bag/box, when compared with the one on the package perviously used, will answer that question for you.

Perhaps this flour is lower in protein? could this be it? Ambient temperatures have been consistent and around 25C

Could be the new batch of flour has less diastatic power, given that the other parameters seem the same.  You can try adding 10-20 percent rye flour, as it tends to have more diastastic power.  Or blend the two whites if you have any of it left.

of the expanded 58% hydration starter.   It takes 24 hrs to double in volume.  Yup. That sounds like slow yeast , I agree.  (If I were to mix up a loaf with those same ratios and it took 24 hrs before I could shape it, I might call it very slow, too slow to be working with AP flour too. It might also be that AP gluten strength has reached its limit with sourdough beasties eating on it and can no longer expand.) Sounds to me like you have a nice variety of bacteria (or maybe too many) but slow (or a low number of) yeast and you want to introduce some faster reproducing yeast.   A flour with more whole grain does seem like a quick way to introduce a wide variety of yeasts fast, yes, much faster theoretically than shortening your feeding time with AP flour.  Time to split your starter and experiment with some of it while maintaining the other one.  (Always keep a backup.)  

1)  Let's add some whole grain,  go outside and look for a patch of long grass away from dogs, dirt roads and weed killers.  Strip some seeds off grasses whack them a few times with a french knife and add them to a clean starter jar.  Increasing your hydration may also help fermentation and help the faster reproducing yeasts get around to the food (think of water as a method of transportation for the wee organisms) so they can get to the food and out number the slower yeasts before those slow-pokes get wiped out with a faster refresh program.  Try 20g starter with 50g water 50g flour and a handful of weed seeds or raw whole grains or coarse rye flour.  At 16 hrs, stir and remove 50g to blend with 50g water and pour thru a small sieve, then add 50g AP  let this starter reach its natural peak and start to fall before feeding again. Repeat the 20/50 /50 feedings or switch to 50/50/50 gram feedings while you play.  

2) take some of your 58% hydration firm starter and add more water.  Feed 50g of starter with 50g of water and 50g AP and time the peak.   Does it still take 24 hrs?   If less, then you might considering working with this starter.  Working to get the feedings closer together first and then firming it up later when you get the peaks down to 6 to 8 hrs.  

You could do this by establishing the peak, for example lets say it's 20 hours.  Feed starter/water/flour  all equal weights (1:1:1) and wait only 12 hrs.  Remove 50g to feed (1:1:1) while the starter is still rising.  The next feed will rise but not peak at 12 hrs.  Stir & remove 50g starter and feed again 1:1:1   or 50g/50g/50g   then let the starter reach a natural peak and start to fall.   It may take 24 hrs but this time there are more 2 -12 hr yeast bugs in the brew and they had the chance to increase their numbers.  Now with each feeding wait for peak and start of fall,  the peaks should be taking less time with each feeding.  If the peaks get stuck on a particular timing all variables being the same, and you want it shorter,  give them a short feed, like discarding at a 6 hr rise and then feed 1:1:1 to let them peak naturally.   A fast yeast will go thru a 1:1:1 feeding rather quickly and easy to compare with other yeast starters.  When you get yeast peaking at 6 or 8 hrs then start reducing the starter amounts in the feeding ratio.  Then after a feed, depending on how the starter is preforming, start reducing the water amounts until you reach your 2o:30:50 (s:w:f)  gram feeds again.   Then you will have successfully changed the timing.

Look out:  if your starter smells/tastes like wet flour (like when you first fed it) when you want to feed it, don't feed and dilute it, give it more time to ferment.  

Good Luck