Ideas on how to monetize my service?

I would start with the easiest and just put a donation bitcoin address while you build on other ideas.

Then maybe have a free version that updates once every 24 hours, and then charge for more frequent updates (every minute?).

If you really want to monetize, charge for it.
Give people a 30, or even 60, day free trial - then charge something, anything, for the product.
Remember... as @amyhoy says: people pay more than $9/month for toilet paper - a product that they literally wipe their ass with. An app that helps them make more money, like yours, is certainly worth that.
That’s the best, simplest option for monetizing as an indie developer imo.
Otherwise, you’re playing on hard mode, and it opens up a whole other can of worms...
Because users that don’t pay, end up becoming the product.
It’s not impossible to create a successful business model with a free tier... but you have to find the perfect idea/circumstance where incentives line up perfectly between users and the business.
Some possible ideas are:

• Ads
• Power features
• Selling customer data
  I’m currently working on a freemium component to the project I’m working on Folyo.
  My advice is to take a look at a comparable successful business model, and steal it whole-hog, instead of trying to come up with a idea from scratch.
  Good luck!

I actually built a very similar app called Trading Dive, but for stocks (and options soon). I actually just monetized the site yesterday. You can take a look at my pricing structure for some ideas or reach out to me if you want to chat further!