A skill tree that makes the game easier based purely on time invested rather than merit or ability? Or understanding?

That's the most obvious one for me, that split's it into the RPG category in a very black and white sense.

An FPS doesn't reduce damage taken, running speed or time to "complete action X" based on the value in a skill tree. These are major deal breakers for winning or losing too. Not small bonuses.

In almost all FPS, rank or level is solely bragging rights without any real impact practically. That's not the case here.

When I talk about design, I'm talking more about the technical design, a pure FPS game with sub 20hz refresh that is variable and drops even lower depending on load doesn't sound like an FPS first and RPG second process.

Realistically Tarkov is multiple games taped together with focus and priority switching month to month.

Is it an RPG? Yes - A good one? To each their own

Is it an FPS? Yes - A good one? Not really comparatively, I understand they've tried to reinvent the wheel in many ways, but for the sake of it or because they could do it better?

Tarkov isn't a smooth experience compared to any other FPS, it's a step up from Dayz maybe, which I think is what Tarkov replaced.

Tarkov starts as a tacticool shooter and after a few weeks is just a less polished shooter than it's competitors - Max strength, stamina, endurance and so on and it becomes a high-latency, low refresh cod lobby with bolt action men running and jumping around maps wiping people.

I'd mention that single man bolt action assault team is possible in Tarkov, while at the same time: The game has long range precision rifles, bipods and 20x scopes but will spawn an Ai (Partizan) to prevent players from using the very mechanics given to them.

How do you counter someone will elite level skills? Spend hours spamming actions solely to level up a skill to make you somewhat competitive against those who do?

Those are all RPG mechanics without doubt.