If you’re going to earn money with your forum, you should be able to pay for a business plan.
Of course you won’t be able to finance it from the very start - like with any other business you will have to do some investing upfront. That does not sound unreasonable to me?
Alternatively, you might want to turn your problem into an opportunity and offer some extra content for free in the first months while you build your user base.
You could also try Memberful which has a free trial Pricing | Memberful.com and it integrates with Discourse pretty nicely.