Hi @Sean Jabbari! First off, welcome to the BP community!
I know there seems to be an unlimited amount of information to learn but the good news is that you already have your first property which is more than most. I am currently searching for my first buy&hold so I know just how difficult that first one can be to find. Despite not yet having a property of my own I'd love to give you some advice based on what I have learned over the past year.
First, you should definitely start by expanding on your goals. Determine what you really want your life to look like in 10 or 20 years and work back from there. If your goal is simply "to get tax benefits and obtain financial freedom," you will have a hard time staying focused and getting anywhere.
Now, it sounds like you already have one successful cash flowing rental so why not just repeat that same blueprint for the next one? I'll try to answer your specific questions but keep in mind I am not an expert, this is just some information I have picked up on my journey from more seasoned investors.
1. I have never heard of anyone calculating depreciation when making a decision on a property. In fact, most of what I hear is that tax benefits should just be viewed as extra benefits, not the determining factor in a deal analysis. That being said, if you're a math guy and like numbers, there's no harm in running them. Then, at least, you'll know what to expect when tax season rolls around.
2. The "right" down payment depends on so many factors that it is near impossible for anyone to give you a good answer here unless they know a lot more about your situation. So my advice here is to decide how much personal capital you want to invest on a property. Keep in mind this also effects your cash flow and CoC return so knowing your criteria for those will also help you answer financing questions.
3. So cash flow is essentially gross rent minus PITI and expenses. There is no best "ratio" that I know of because that would completely ignore taxes, insurance and expenses. However, when I do my calculations I look for $200/unit/mo cash flow ($100 is my minimum for multifamily). The fast rule for cash flow is the 1% rule and this can be used to quickly determine if you want to take a closer look at the property.
4. Interest rates are historically low right now so it is a great time to buy but the coming downturn will also be a great time to buy because homes will be significantly discounted. So the real answer is that asap is always the best time to invest (intelligently) because the sooner you start investing (again intelligently) the more you will make over your lifetime.
5. There are a billion other questions you could ask and that is what this community is here for: To help each other. Just be sure that your questions are specific enough to get a specific answer. If the answer can be found by googling or searching here in the forums, you will not likely get a response.
And please if you have any questions or just want to chat, feel free to message me anytime.