No offence, but the fluctuating difficulty isn't an issue. It's the halving and constantly dropping exchange rate that is making mining this coin mediocre.

Your calculation is also incorrect. $5.15/day sounds like something from whattomine calculator. In fact it is hardly $3.00/day with one 1080 Ti (net profit). I stopped mining SIGT at the very moment the halving was applied. Until then it was a joyride, but now SIGT is not even in the TOP 10 most profitable coins. And believe me, $5.15/day is just a beginning of what you can get out of your card. You just need to do your own research and remove whattomine from your bookmarks because NONE of the most profitable coins is listed there.

I still keep something around 2k in SIGT hoping to dump around $0.12 but the depreciation is showing signs of a long-term trend.

If you care about SIGT, support it by mining it.
If you believe in SIGT, mine more profitable coins and turn your profit into SIGT.
If you care about PROFIT, do your homework and find yourself a more lucrative coin to mine.