Looking back at Rivals, I see 2 ways to play it,
1) Half in - Just get the L1 Alligator swamp (16 quests), I think that is doable for most and shouldn't cost any diamonds. Even level 1, it is a very good building, red/blue defense which most cities need. I don't think it is possible to get to professional league because those doing the full rivals will move that top 5% bar way up there. You do walk away with some extra boosters too. You should also get gold animal crossing upgrade, 550 paws, but you still need to play it daily using your timer tickets. If you complete Q17, you get 3 tickets, those really help to get 550 progress. Do it 3 times, 9 extra tickets, don't skip it, makes a difference.
2) Go For the Gold (-en tickets) - all in on this one. Getting this one complete has a high probability of spending some diamonds unless you really balance those tickets/coin in quests. I spent diamonds, I always spend some on this event. Completing rivals was easy, WHEN YOU SPEND for coin. I did NOT need to spend for tickets. You come away with lots of boosters and 2 - L2 buildings, especially L2 alligator swamp. I have 3 of them, looking like the Florida Everglades in my city...lol Coming out with 9 golden tickets, lots of progress there. My rule of thumb (FOR ME), 3 good games, rush it, 4-5. I say good games bcs I spend seconds looking, maybe get some better combinations but I don't pause too long. Those seconds add up and I am usually on the edge of 3/4 games.
The 3rd round of rivals, Q23 went from 3 to 2, 6-star chests, that helped a bit.