Should I implement a subscription system like them from the very start, or should I focus on getting traffic first keeping the platform open?
I've been through this with a few SaaS businesses and I'm doing it again with another at the moment.
I'd recommend building your paid subscription implementation from the start. At the very least you should have pricing tiers visible and allude to upcoming features.
There are multiple reasons for this approach.
Firstly, the "perpetual freebie users" are generally awful users and not something you want to attract. These users will never spend money with you, even if your service is brilliant in every way, and they will moan when you restrict/remove free tier features. There is a (small) argument to be made that before you have any real traction any voice is useful, although in my experience this is rarely a consideration, unless you are in a race to build the next Tiktok.
Secondly, the value perception of your service is incredibly important. It's hard to understate this. You may have a segment of users who are willing to part with $ for a service, but as soon as you introduce it as a free offering then that's how it will stay in their mind.
Thirdly, parting with $ - however small an amount - is an incredibly strong signal from your users. That means you are doing something useful which can be turned into something larger, and you can charge more for it. For example, we offer a $9 tier which is really just the gateway drug to our $59 tier.
The final point is that you should start it now because subscription logic is hard and will take you longer than you expect, although this varies with the platform. Apart from anything else the headache of collecting VAT/Sales tax (particularly with the EU's place of supply rules) will take you some time to implement.
Good luck.