It's not necessary, you could have a complete experience without it for sure. But I will say you feeling like that about season 3 doesn't surprise me at all, and it was a risk the team took that I personally loved but I can totally understand why people wouldn't like it.
I'll put it this way; the movie fills in the gaps that season 3 introduces, and there are story beats in the movie that I think only truly hit with the context of season 3. But if you didn't like 3 that's totally fine, tonally it's a mix of case 1 and case 3 of Sinners of the System, if you liked those two movies (not sure if they technically count as movies lol) you're probably gonna like the movie.
I could go further in depth but it will spoil things, just let me know and I can go more in depth :)
Oh hell yeah, just checked them out and I really fuck with it. Love the screams, very raw sounding, hopefully they do get bigger because I'd love to hear them live myself