when should i care about monetizing?

As long as you're realistic up front about it then there's no problem proceeding. Just understand the revenue and business side of things is unproven and questionable. But if you do successfully build a large audience you may be able to monetize it eventually. Maybe keep it as a side project and let it grow slowly over time so that you don't have immediate pressure for it to turn a profit.

Validate first and then try to monetize as early as possible (even before the product is ready, if possible)

I won’t say either or, I think the answer is in the middle. Grow an audience you know it can be monetized, sure you don’t have to know exactly how or what method, but the motivation of users must be there from day one... and I think that’s crucial as well for hiring experts, as it will be the first question they will ask you in this form: “sure, I share your vision but how can I know you can pay my salary in the long run?”

Should be part of your design

I would get it out in the public ASAP if you can and try and drive traffic to the site then monetize additional features later down the line if there is an interest. It might become more apparent how to make money from it once you have some traffic. Good luck with your product