Are my pitch rates bad?

[gundog48]

I'm having serious problems with doing generational fermentations with my rum. My process has been going:

-Create a 20L molasses wash
-Add a suitable amount of yeast (I've been using a packet of commercial slurry)
-Add a small amount of that yeast to 1L of 1.06 molasses wash and allow to ferment on stir plate as a starter for the next batch
-After about 48h, refrigerate starter
-After about 72h, the first fermentation has completed, SG of about 1.015
-Rack the wash off the lees and strip
-Weigh out the same amount of molasses for the second distillation
-Dissolve in 4L of hot dunder (for 20L batch)
-Add fresh water to the yeast remaining at the bottom of the fermentor, then add the molasses-dunder mix. Temperature does not exceed 35°C
-Aerate wash with an aeration stone for 5-10mins
-Pitch the starter, retaining a small amount for the next generation
-Second fermentation will take off rapidly, drop from 1.08 to somewhere between 1.04-1.05 and fermentation will halt at a pH of 4-4.5

Each batch has an SG of about 1.08. I'm using dechlorinated water, adding 4g DAP, 2g at start and 2g after 24h. Also adding 2g epsom salts. I'm adding a vit B complex as well as a multivitamin. I'm keeping the previous generation yeast alive and pitching fresh yeast. I'm using Hornindal kviek yeast which is also usually very tolerant of low pitch rates.

From where I'm sitting, I'm giving this fermentation the best possible start in life, so what on earth is happening? Is the remaining yeast in the fermenter too stressed to be useful, and a 1L starter not sufficient? Am I messing up elsewhere? I've never had such trouble with generational fermentations before! I've tried getting a starter going by diluting the current wash 50% with water, but it won't get going. I'm pulling my hair out here because it seems simple, yet I've tried this process about 3 times and my failures are very consistent!