I previously posted a reply about whether Destiny 2 is easy plat with just the free PS+ base game, not spending any real money on the 3 expansions. The answer is no, not easy plat, due to a trophy for completing a raid or nightfall in prestige difficulty, which is very hard.
Now with the 3rd expansion (Forsaken) out, the platinum is unattainable (at least temporarily) regardless you buy the expansions or not. One of the trophies requires completion of trials, which is not scheduled until Season 5, speculated to be Dec at the earliest. Some other trophies are now broken - like completing Shaxx's Call to Arms (which is unavailable), completing flashpoint (you can do it but the trophy just doesn't pop). Not sure if these are temporary.
More importantly, with Forsaken, the aforementioned prestige nightfall and raid now has a prerequisite light level of 330. You can only reach light level 300 for characters at level 20, which is the cap for the base game. So the platinum trophy is basically locked behind a paywall. You cannot get the Destiny 2 platinum trophy without paying for expansion DLCs.
And if you want to buy the expansion DLCs (having owned the base game already from PS+), the 3 expansions add up to us$60. If you buy the complete bundle (base game + 3 expansions), it's also us$60. So the base game is basically free anyway. Bungie/Activision giving the base game free on PS+ is just giving it as a demo (they have already given access to the same during free weekends).
As I posted before, Destiny 2 base game is still very fun to play through, solo or with a few friends, just doing the story campaign and the 'adventures' (side missions, you have to do some anyway to grind level before continuing with story missions). Just so long as you play once (or 3 times if you want, doing it with different character classes, as they do play rather differently). But don't buy the expansions unless you are committed to the daily and weekly grinds for the long term.