Every questions you have make sense and they are essentials when setting up your company. Let's address them one at a time, Miiiki.
Your business started last July 2017 and let's make sure you have an active QuickBooks Online subscription. This would allow us to address your concerns as we go through one of them.
When you connect your bank account, QuickBooks Online can only download 90-days worth of bank transactions. Some banks allow more than that. If you need more of these bank transactions to be downloaded, we can use a CSV file and import it to QuickBooks Online. You can use these links so you can successfully do the import process:
Once you have imported the bank transactions, you can categorize them. That includes the tax payments you've made on the previous years.
For your second question, you are correct. You can set the Opening Balance on December 31, 2019 if you want to start setting up the account within this year. Here are some links that you can use for reference:
In QuickBooks Online, we can set up the depreciation of an asset. Here's also a link that will serve as your guide: Depreciate assets in QuickBooks Online.
Though that is how we set up depreciation in QuickBooks Online, we'd still recommend consulting your accountant. They know what's best option based on your company setup.
Now, let's address how you can track your income. Based on the details you've provided, using the Class Tracking feature would your best option. You can either assign these classes per transaction or per line item. Here are link about it:
Lastly, we only turn on the multicurrency if you want to track and receive transactions with foreign currencies. Wit this, you'll be able to track specific transactions with their respective currencies. To know about it, here's a general link about multicurrency: About Multicurrency.
This article has multiple articles relating to multicurrency and everything you want to know about the feature. Before you turn it on, you can also reach out to your accountant. They might be able to guide you whether this option is best for you or not.
As you go through everything that I've mentioned above, keep in mind that we're here always ready to help you more. Let us know how this goes so we know where we can start.
