Also, how often you collect tributes and, as you mentioned, the percentage chance of tribute for each kingdom. This percentage is increased with kingdom power but also with Red guild statue, to a maximum of 6% boost at level 100.
Arguably, you could “farm” gems in Treasure Hunt. Or do so by opening gold and glory chests. But by far the best way to get gems (short of spending money) is from collecting tributes every hour.
I’m an early end-gamer, so I still have a long way to go. But I’ve been playing for 2 weeks short of a year so that might give you an idea. My income is 1113/h and I have 5 kingdoms at PL 16. I still have 20 kingdoms at 9 since I’m missing a bunch of mythics and/or copies of troops.
That said, I get an average of 14 gems per tribute, so ~200 gems a day from tributes.
EDIT: But the key thing that gives you more resources is playing more. There are plenty of people that list 300k/hour as their general farming income. With my VIP 0 bonuses, that amounts to 60 3-trophy pvp fights, 50 explore 12 battles with gold team, or 200-250 fast explore 12 battles.