I don't really understand this point. Why do you think that angling at 20 degrees will give more generation on cloudy days? This isn't something I've noticed when I carry out light readings.
When there is cloud cover, diffuse light is coming from the whole sky.
The more the panels have a direct facing to the light sky the better they can generate.
I've also noticed that my array's best days of generation are sunny mornings and white cloud or partly cloudy afternoons. The white cloud being slightly luminous and much brighter than a blue sky - somewhat reflecting the light around and benefitting my panels if the sun is too far round to shine directly on the panels.
I know someone with a peculiar arrangement of 2kW North and 2kW South (they've even had a solar installer come round and say "you've been mis-sold because you've got 'em on the wrong roof!") but their array is performing ridiculously above expectations due to about a 30 degree roof slope (maybe a little shallower - perhaps 25'), allowing those North-facing panels to point much more up to the sky on cloudy days which make up the majority of British weather.
Last I heard from them, about a week ago, they had gone through 2100kWh between mid-May and late-November (both the generation meter and inverter logs confirm this).
That's 2100kWh from a part-month of May &November, and full months of June to October, with a 2kW North + 2kW South array at 30deg. Their huge outperformance so far relative to expectations appears as if they'll get to around 3100-3200kWh total generation in their first year.
Performance has matched my 3.75kWp ESE array with 40degree slope.
Their inverter and panels same as mine.
Furthermore, they never have a really bad day, and never have a massive day (my panels are much more extreme from one day to another - tenfold variation from one day to the next can be seen sometimes from my array) - partly due to their split array, but partly due to those shallow panels pointing at the light sky even on days when there is no direct sun.
In summer, for example, I could get mid to high 20's kWh per day, but they never got above low-20's. However, on the many dull days they performed much better than me, with far fewer low generation days.
Also shallower angles tend to be better at catching some sun at any time it may appear, whereas my 40-degree angled panels give almost-unusably huge generation on sunny spring/summer mornings but very little in sunny afternoons nor on cloudy days as the panels face almost as much towards the dark landscape of the horizon rather than the sky.
