As you said, it's not the most popular domain extension.
Some TLDs have a terrible reputation, I mean not just old TLDs, but also some of the newly released TLDs. .bid for example, which seems to be used exclusively for spam. I even blocked it on my mail server. Nothing to lose and everything to gain, because I don't expect any legitimate mail from a .bid address, and none of my regular correspondents use it. I also blocked a couple other TLDs and I know I am not the only one doing that.
Regarding .tk I would not be surprised if it's blocked in some corporate environments at proxy level. Generally speaking any TLD that is available for free or a low registration fee is abused more.
To make an informed decision you might want to check the stats from Spamhaus:
The 10 Most Abused Top Level Domains. .tk is not in the top 10 nowadays but many of the featured TLDs are Freenom TLDs, that you can get for free and this is not by chance that they end up in that hall of shame.
On the other hand, registering for 5 years in advance sends a positive signal to search engines that you have long-term intentions. But if you are willing to pay, then I would encourage you to consider another TLD that carries less stigma. The question is, why register a .tk when you have plenty (hundreds) of alternatives?
Notwithstanding all the above remarks you could go ahead with your project, buy a .tk domain and you might still achieve good SEO but if mail servers do not accept your mails because they have a blanket ban on .tk, then your domain name is kinda useless. I would think about all the other practical aspects, not just the SEO.
I am not saying that this kind of blanket ban is prevalent, but there are people actively looking to ban .tk as you can see here or here. Since you are asking specifically about .tk I have the feeling you are maybe making life more difficult for you.