You are correct that you can take the reboot films and STD off the table and dismiss with prejudice right from the jump. The short answer is everything that isn't that stuff is some degree of canon.
With real Star Trek, it's been my experience that among most autists, TOS films (except ST5), TNG Season 3+, and DS9 are generally regarded as firm canon, barring obvious mistakes and fuckups in the writing which can put individual episodes in this purgatory soft-canon category where you perhaps can accept the overall story and the characters, but plot details, bits of dialogue, or aesthetic elements are overlooked on a case-by-case basis.
TOS, TAS, ST5, early TNG, Voyager, and Enterprise are generally soft-canon. The specific shit that they tried to establish in the episodes should be taken or left with no guilt and zero fucks given. TOS is revered, but also clearly a work-in-progress with iteration happening all the way through the first 2 seasons of TNG. Don't let sentimental fuckheads try to defend TOS as entirely hard canon because it just isn't. That kind of thinking is what got us the retarded Klingon retconning in Enterprise.
The TNG films I guess are also soft-canon, although you end up having to overlook so much fuckery that personally I'd just as soon forget they exist. I never re-watch them and don't miss them. Reasonable autists disagree, though, about all of the "prime" material so my advice is to focus on the good stuff and ignore the rest. It's not like they're ever going to make a post-Voyager/Nemesis series anyway so it's probably inconsequential.
Are you working on a fan project or just want to know what to watch?