Congrats on finishing your game!
Don't be intimidated by the publishing process. There are a number of steps, but they're mostly straightforward.
I'd suggest you don't actually publish it to the store until you have your marketing materials (video, etc.) done, though. How well your app does in its first few days can make a big difference in where it shows up in the store listings, so if you're planning any marketing effort at all, be ready to kick that off before you publish it.
The publishing process is similar on iOS, but with more hoops to jump through and a bit higher cost. Also, Apple charges USD 99 per year for their developer program (vs a one-time fee of USD 25 for Google Play), and Apple reviews every app before it appears on iTunes. Once you submit an app it usually takes a few days of "waiting for review" before you get a result.
It's common to have an app rejected for something subtle, especially with your first submission. For example, don't have a "Quit" button to close your app. The iOS user interface guidelines advise against that, and Apple will likely reject an app with a quit button. They want users to always use the home button to leave an app, and they feel users might think the app crashed if it went away for any other reason, evidently even if they tapped a "quit" button. The lesson here is that you should read their user interface "guidelines", because they're actually closer to commandments than guidelines.
As far as I know, you still need a Mac to run Xcode to build and upload your app package for iOS. If you don't have a Mac, the cheapest route would be to buy a used Mac Mini, but if you go that route, make sure you have enough RAM. Xcode seems to hog more resources lately than it did a few years ago, and dropping some extra memory into my Mini helped dramatically.
Publishing first on Android makes sense since it's faster, cheaper and a bit easier. As for monetizing, I find that while I get more downloads on Android, I tend to earn a bit more from iOS users. I'm monetizing through in-app purchases though, and iOS users are known to be more likely to pay for things than Android users. You may find that ad revenue is in fact better on the Android side.
Once you have your app on Google Play, and you see how it's earning, take a look at Amazon Underground. If your game is something people will tend to spend a fair amount of time on, the revenue there can be reasonable. You give people all the features for free, but get paid USD 0.002 per minute people play. If that's more than you earn on average from Google Play users, putting your app on A.U. is an easy decision. If not, it might still be a good move, but it's not as simple a decision. In my case A.U. makes up a pretty small share of my income, but it took very little effort to get a couple of my apps on there.
Good luck publishing your game!