I gave up on DIY scorpion islands a while ago when they nerfed the tarantula/scorpion spawn rates. Sorry, I wish I had some useful information to help you out regarding this.
As for other ways to make bells, I've been getting a decent number of sharks and dorados on my island lately, and that's currently my best source of bells that I can make from catching things. I know it's nothing like the glory days back in March when we were walking around with pockets full of tarantulas, but you know, the wheel keeps turning.
To me it's not as much fun, but you can do pretty good in the Stalk Market if you're part of a decent network of online players. Personally, I don't like to play online with strangers but my wife and son also play the game, and between the three of us so far we've always managed to make a profit.
Also, here's a link to a calculator that will predict your turnip prices for you, if you enter the prices that Timmy and Tommy offer you during the week:
Animal Crossing - Turnip Prophet
turnipprophet.io
You can use the calculator to maximize the return on your turnips because it tells you in advance when the best prices will occur. The calculator doesn't return an exact price, but will give you a likely range of prices. The calculator uses information from the previous week to guess the turnip prices in the current week. As long as it gets good data, I've found that the calculator is very helpful.
That's all the info I have to share, good luck with that mortgage!