MVP? Landing page? Customer interviews?

Your MVP seems too big if you've been working for months, why don't you focus on validation instead?

1. Short answer: It doesn't really matter. But what you need to know is why would a customer choose you over other alternatives in the market?

2. Your validation seems indirect. Have you ever spoke to a real prospecting customer?

there's already a hundred-million dollar company doing something similar, but there's this niche they're not serving that I want to focus on

That's awesome, but that niche needs to be validated too, and the validation is the beginning of marketing/sales, since it gets you your first customers.

Also, there's another company with an almost identical product to mine, but they're limited to Western European customers and I'm in the US.

Even better! :)

Does the fact that these companies are already doing something similar (and at least one of them is very profitable mean) that I don't need to focus much on validation and should continue with MVP development? Or does that not matter since it's only 2 companies and I should still go through the whole validation cycle?

I think you should be doing the validation anyway, because getting the first 20 paying customers is the result of your validation, and you are going to do it before or after having a product. Now, in your case, if your product is very close to be useable, making it useable might be a better path, since validation is easier when the product exists.

If I should validate further before continuing with my MVP, then I plan to reach out to some well-regarded community members on Reddit and basically do The Mom Test with them. Would that be enough validation or do I need to go for a landing page and collect emails as well? Do to the nature of the product it's free-to-use, so I can't ask people to buy or pre-order it.

I don't think the landing page is the only way. Collecting email addresses of the people that feel strongly about using it, if you are strict about not manipulating them (the mom test), then that's probably enough.

If you are solving the problems for the US customers, then you should have no problem finding and talking to your potential customers to validate your biz.