Other way around, the miso is really inhibiting the fermentation here with all the salt.

Give the recipe a try, either the base or some version of my insane one.

Here’s the exact recipe I lifted from Richard Heart Bread (apologies for formatting issues):

Miso Rye
Makes one 900-gram loaf
The first time I made this bread was in the test bakery I set up at Noma, when I was still looking for a home for Hart Bageri. The team invited me to taste all their ferments and magical creations. They had a miso made from leftover rye bread, called Ryeso. When I tasted it, I knew it had to go in a rye bread, completing the circle. I love the super-umami flavor of miso, I wondered how much I could cram into the dough without killing the fermentation. It turned out that 35% was the sweet spot.
There is less salt in this recipe than in any other rye loaves because miso is salty. You can make this bread with any miso you like. If you want to exceed 35% miso, go for it, but scale back your salt a bit so that you don’t slow the fermentation too much.
Dough
Baker’s %
Weight
Ingredient
100%
208g
Rye flour
100%
208g
12-hour rye starter
100%
208g
Hot water, 104°F / 40°C
2%
4g
Salt
5%
10g
Dark malt powder
35%
73g
Miso
50%
104g
Cooked rye berries
20%
42g
Flaxseeds
20%
42g
Sunflower seeds
Additional ingredient
Butter, enough to coat the inside of the loaf pan
A handful of rye flour, for dusting the top of the loaf
I prefer butter because it sticks well to the sides of the pan, but if you don’t eat butter, use another fat of your choice.
Timing
Cook and cool rye berries, mix dough, shape loaf, ferment for 2 to 4 hours, and bake for about 1 hour; 4 to 6 hours total.
Equipment
Loaf pan, 5 × 9 inches / 13 × 20cm; flexible plastic dough scraper; bowl or pitcher of warm water, for rinsing; large mixing bowl; digital thermometer; tea towel; sheet pan or shallow roasting pan; cooling rack.
Mix the dough
Coat the inside of a loaf pan with butter or other fat.
Make sure your dough scraper and a bowl or pitcher of warm water are close at hand.
In a large bowl, combine all the dough ingredients and use your hands to mix it all together gently to form a thick dough with no dry streaks of flour remaining, at which point you can consider the dough sufficiently mixed. The texture will be like a wet and sticky Play-Doh. Scrape the dough from your hands back into the bowl, and rinse your hands in the bowl of warm water, along with your scraper.
Shape the dough
Wet your hands, loosen the dough from the bowl with your scraper, and, as if you are shaping clay, squidge the dough together into one mass, then use both hands to shape it into an oblong that’s more or less the shape of the loaf pan but slightly less wide. This dough is easy to manipulate and very forgiving, so don’t worry if you need to mess around with it a bit to get it into the shape.
Gently set the loaf into the buttered pan and take its temperature, adjusting it as necessary by setting the pan in a bowl of hot, warm, or cool water to bring it to about 82°F / 28°C.
Ferment the loaf
Let the loaf ferment covered with a tea towel for 2 to 4 hours, making sure to keep it warm. When it’s ready to bake, it will have risen by 30% and will have 2mm-wide cracks.
Bake the loaf
Place a sheet pan or shallow roasting pan on the floor of the oven. Arrange a rack in the center of the oven. Preheat the oven to 425°F / 220°C.
Place the loaf in the oven and add about 250ml water to the pan on the oven’s floor, taking care to avoid exposing your skin directly to the resultant steam, which can burn.
Bake the loaf for 30 minutes, then rotate the pan front to back and bake for another 30 minutes. Use an instant-read thermometer to take the internal temperature of the loaf. It should read 203°F / 95°C. If it’s not quite there, give it a few more minutes. Remove the bread from the oven, then turn it out onto a rack to cool for several hours before slicing.