buying links when you don't know SEO is not a good idea. You'll probably not see any results from that package, not because the package is bad (I never tried it, so I don't know), but simply because you don't understand how backlinks work, how to use them and where. You've probably already wasted your money....
Regarding the indexing, yeah indexing is hard nowadays, with the pandemic going on everyone and their little brother jumped into SEO and this has created a lot of competition and my guess is that this is one of the (less talked about) reasons for which google refuses to index content (too much competition that steals rankings from big-budgeted sharks who keep google happy).
Furthermore, web 2.0s are even harder to index nowadays than they used to be only a couple of years ago, and this is because they're also devalued by google (web 2.0 backlinks rarely impact your rankings anymore, if at all)... Besides, many web 2.0 sites nofollow your links, noindex your posts (see medium.com), shadowban your posts, or outright delete them. If half of those 500 web 2.0s are not removed / nofollow'd / whatever 6 months down the road then my name is Elon Musk... and it's not!
your options are still numerous even at this point. But I would start with hiring someone to do site audit, competition analysis and keyword research for you before spending any more money on backlinks. Either this, or learn how SEO works so that you know how to rank because blindly throwing money at backlinking packages without knowing which pages to point those backlinks to, how many of them to build, for how long, and with what anchors will most likely tank your site before you can figure out what happens.
Can I use guest posts instead of web 2.0s for a new site? Is that safe for a site with only 35 backlinks?
yes, you can use them. All backlinks that are good (or at least not spammy) can be used for any site, at any time. The problem is knowing where to point them, what anchor texts to use, and to make sure that you drip-feed them
