Hmm, firstly I'm obviously not any sort of expert. Secondly, this is me running off memory so I'm quite capable of getting my 'facts' wrong. With that in mind:
The 'total deaths' thing could
possibly reach the billion mark.
Very possibly. It'd depend on how you count.
The obvious one is direct deaths and incidentals from collateral damage (waves, stray bolts of lightning, Simurgh screwing with people). This actually seems like it would be the lowest contributor to me.
But then you could include suicides of people who think (fairly rightly) that the world is fucked, or that the Simurgh got to them or whatever. I'd argue that given the state of the world, most (but not all, silly parahumans) of which is caused by the stress of Endbringers being a thing, that there would have been a large increase in suicide rate since Levi showed up (Behemoth originally being considered just a strong cape that would eventually be put down, or something).
Now include environmental damage, which would effect food production. Every time Behemoth irradiates somewhere, every time Leviathan changes the terrain (not to mention the effect his waves/actions have on the oceanic ecological systems), and every time places get closed off because of the Simurgh (again, not to mention the additional resources directed to those places to keep them shut up/supply the people in and around them). This puts additional strain on the economy, which also increases suicide rates, food shortages, food costs and animal populations, all of which result in more deaths.
Birth rates are similarly changed because of their actions, I'd assume.
I can't remember where I saw it, but I'm pretty sure it was mentioned that Earth Bet has a population of somewhere in the 5-6b range. We were around 7b at the time of canon, so Bet's lost (
very roughtly) between 1 and 2b people. It seems to me that it's likely the Endbringers are responsible (directly and indirectly) for at least half of this, to go along with all the death parahumans have caused.
Didn't you say this was your first time trying to write anything? Going ok for that.