Hi--
You're not going to like my answer. But I'm not going to blow smoke up your a$$ either.

The smarter play is always the longer, repeat revenue. This rules your one-off purchase out. You can still turn the purchase into recurring revenue by making it a SaaS. But the world is a SaaS graveyard at the moment. This leaves you with a few options, none of which are listed above. But first, some reality.

1. This ain't Field Of Dreams. There is no "build it and they will come." You are over-simplifying if not completely neglecting marketing, especially when you're aiming for $20k per month. You are an email marketing wizard and have some SEO knowledge, great. I get, on average, an open rate of 47% on a cold list, 76% on a warm. Let's assume yours is somewhere between the two since I don't know if your list is cold or warm. At $50/purchase, assuming a 20% click rate, and 5% conversion rate (all super super super generous), you'd need a list size of fresh emails (<3 months imo) of 8,000. For one month. For one year of business at 20K per month, assuming you can maintain the stats, you need 96,000 emails, assuming you can maintain those killer stats, Mr. Wizard.

2. You're not the first person ever to have a unique solution for ___Insert Problem Here______. That being said, you didn't acknowledge your competition. Who are they? How big are they? How dense is the competition in your space? And I don't mean competition from a "well, no one has a product like me so I have no competitors" perspective. I mean who has your target market's attention in this space? What are they up to? How are you different? What's your *gasp* competitive differentiation?

Basically, you're asking an EXTREMELY complicated question with no context and you've boiled it down to irrelevant facts rather than focusing on the business viability.

Oh, there's also no such thing as get rich quick. Sorry, kiddo.

So there ya have it. The truth like no one else will give it to you. That being said, here's my gift to you. This is how you make this pile of dog crap into a business:

Path 1:
1. Get ready to do some work. With no money to market or advertise, you're relying entirely on organic social, email, referral, affiliate, SEO, word of mouth, and partnerships. That's what's in play. I can't make a recommendation as to which channels are the most viable without knowing more about your target market.

2. Take your knowledge product, split it up into modules and put it into the open source Canvas LMS or Moodle.

3. Get a friend with some design skills. This is important. Illustration is preferred. Illustrate or at least design the hell out of the site and modules.

4. Seek the advice of an attorney, as you will want to protect your assets. Otherwise, if they are that valuable, they will be stolen and you will never be able to make a buck.

4. Or, offer the first module in beta for free. If it's good, people will subscribe once they know it exists.

5. Begin activating the channels from number 1 based on market engagement.

6. Hire a professional. You may be a super expert in your field, but you need some business wisdom to guide this.

Path 2:
1. Get ready to do some work. With no money to market or advertise, you're relying entirely on organic social, email, referral, affiliate, SEO, word of mouth, and partnerships. That's what's in play. I can't make a recommendation as to which channels are the most viable without knowing more about your target market.

2. Give the knowledge away for free.

3. Build a personal brand.

4. Begin selling tangentially endorsements.

5. Become Neil Patel.

6. Begin activating the channels from number 1 based on market engagement.

6. Hire a professional. You may be a super expert in your field, but you need some business wisdom to guide this.