I think you need to size for a 10 year historical storm with a climate change rainfall intensity factor (40% in my case) so 3m³ soakaway seems a bit small. I have 10m³ for about 200 sq.m roof, good infiltration rate. You don’t need to have it all in one place, I have four, roughly one at each corner of the property. I am convinced this is a better solution anyway, more surface area for infiltration and shorter drain runs. My drainage engineer has retired unfortunately but I found him (and other ideas!) by looking through local planning applications.