As what zig said, I stopped spending when mistress manicure came out - i let my monthly expire and almost never bought again apart from the occasional guild gift (still very rare) and I think I bought a deal a few months ago.
My main source of diamonds is the daily pit and coli rewards, I'm lucky my timezone and my job gives me the flexibility to compete there almost daily (unless i have a meeting or if i'm in a call).
Get the 3 free stamina daily (12, 6 and 9pm), buy stamina 4x for 50 diamonds daily. Hoard stamina, don't use any until a weekend contest with double drops. You'll have 10k stamina in about 3 weeks, use them weekly or every 2 weeks (every scrap collecting contest) if you want. Buy stamina from guild shop, buy crystal shrines and stamina resets from bazaar. Buy 10 raid chests 4 times max (stop if you get an 80 pack before 4), you'll get at least some stamina and double drops tickets if you don't get any 80 ticket packs.
You can seriously keep up or even do well with the right resource management. You won't be top power on your server or have all the latest heroes, wait and only work on the good ones based on feedback from your guild or the community. Work on your runes (see crystal shrines) so you can compete for rewards and not lose at all in war, get some defense wins too.
As an experiment, I started a vip 0 account on s11, i gave in and made it vip 4 when i reached lvl 100 because I don't have enough time to not do raids. It's now lvl 112 on plat 1 on both coli and fight pit for my diamond income. I can't even spend all my diamonds because I keep running out of stamina refreshes. Just need time to catch up and i'll be getting on challenger daily on s11 too.