In my experience, it's all about grind size. If your target brew time results in too bitter a brew, I don't see how you would avoid a shorter brew time for these beans. If it's hard to hit the brew time you are aiming for, you might have success with another grinder. It shouldn't be necessary to touch brew ratio and temperature unless you're really fine-tuning taste.
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5 pours might be too many. Each time you pour, you remix the bed of grounds, letting fines settle into the filter more. In a perfect world, most of the filtering is done by the grounds themselves.
Darker roasts extract more easily. I would definitely try a lower temperature. I have the plastic V60, though, so my temperature doesnt get affected much. You could try preheating your v60 first. My lazy method of preheating is just putting the v60 on top of my kettle as it's warming up. You could do that without the lid on your kettle to really steam it to temp.
Darker roasts are lighter, so 15g of a darker roast are more beans that 15g of a lighter roast, so you could decrease dose, but you are probably fine there. I dont think having a too high dose should make it bitter.
A rule of thumb I recently heard is that you should use less water with darker roasts, and more water with lighter roasts. This could be reversed if your water measurement is a constant, so use less grounds with darker roasts and more grounds with lighter roasts.
The reason for this is that the darker roasts are more water soluble, so they will extract with less water.
To me, dark roasts are all about the bloom. I also think 5 pours might be too many, but YMMV. I wouldn't grind any smaller lest you enjoy lots of fines.
I bloom 20g of dark roast with 35-40ml of boiling water for 90 seconds and then I do 3 100ml pours every 30 seconds. This is my everyday V60 coffee.
This video goes over the main tweakable parameters of a pour over recipe. I found it to be a good guide for tweaking a recipe.
The darker the roast, the lower the temp. Ultra Light ~ 99-97C; Light 96-94C, Light/Med ~ 94-93C; Med ~ 93-91; Dark ~ 90-88C; generally but these can vary also.
5 pours I find generally over extracts and often clogs a filter due to an overly disturbed bed. I try to reduce pours and pour as gently as possible.
I don't know what "higher roast" is, but if you're talking about dark roast, then 100°C is pure madness!
I brew my dark roast at 85°C using a ceramic Hario V60. I do preheat it though.
With that said, I don't recommend a specific temperature, but you should perhaps not be afraid to experiment with lower temperature.
And don't forget to check out Lance Hedrick on YouTube. He have a tons of tips and recommendations for the best brew that you can make with your equipment and your water.
5 50 ml pours will give you maximum extraction, but sometimes maximum extraction isn't what you want, particularly with something high in tanins. if i'm making coffee for enjoyment rather than trial i'll do a bloom until the bloom has drained and then pour to 200ml, wait for that to drain mostly, the pour to 250ml