I should clarify -- some of the loss is indeed in the conversion to USDT -- my accounting is done that way daily due to (annoying) US IRS regulations.
In pure ZEC, I've gone from an average of 1.6-1.9 ZEC/day (7/17/2018) to an estimate of 0.9 today (09/8/2018).
So yeah, not halving every 2-3 weeks, but a marked drop and a continuing downward slope.
I have 12 Z9 minis running currently (~192-194kSol/s pool side) and have 3 Z9s on the way (in theory, if bitmain every ships). I don't regret the 12 Z9 minis, but I'm unsure on the Z9 large units.
You referenced the difficulty chart, which as of this moment shows a 28% 1 day difficulty change, and a 58% 30 day difficulty change. While I realize there is a lot of fluctuation in the rates in a given moment, model out 60% month over month and see how fast your returns disappear. Bitmain is going to keep shipping these until people quit buying them and I expect that the units in flight over the next 30 days will at least add another 60% difficulty...
-j
When I look at my GPU gains, I still think that it was a very good decision to get Z9 mini ASICs. I have only four :-)
Please remember that Zcash difficulty is adjusted according to hashing power and there is nothing like Bitcoin's dramatic difficulty change. Also a lot of GPU systems were mining ZEC and now they are out.