Looking for large keyword database covering all keywords

but not too large

I want to compile a list of quality forums via serp api. Hence the keywords should not be too specific, sth about 10M keywords would be fine, covering all niches. Where can I get such a database?

good:
dogs
dog food
nutritious dog food
unhealthy dog food
dog care
dog health

not good:
dog care in XY at foo in front of bar when sunset