Alecsandros wrote:"...Jaspers mentions an average rate of fire over the entire length of the battle time...."
PG average RoF was 37,5 secs (1,60 rpmpg) over the 14 minutes battle.
Jasper says that "the heavy artillery attained a salvo rate of fire of 27-28 seconds", so this seems to be the max RoF during the battle.
you wrote: "I also think the final part of the battle (6:04 - 6:09) was done at a lower rate of fire....."
This is only possible if the fore turrets were not wooded. In case the turrets were wooded after salvo 28 against PoW (count of ordered shots is now 48 initial "ranging"" + 96 during the "rapid fire" from 5:59:30 till 6:04:30) PG had to fire 36 shots in 4 minutes and half, thus the RoF (with just 2 turrets bearing) is still 27/28 seconds.
Only in case the fore turrets were still firing when on course 270°, then we can assume Albrecht slowed down the RoF, but this is not what Jasper said: "...Albrecht continued to fire rapid partial-salvos...."