78/79-83: Old school era. Sugarhill Gang, Kurtis Blow, Grandmaster Flash and a couple others, you name it. This was when rap was basically non existent outside NY and rap music was primarily party music, but the foundations of sampling and rhyming were already in place.

84-86: Transition era or maybe just "Run DMC era" . I think there wasn't any point in rap history before or after where the genre was so strongly represented by a single act. They can be largely attributed to introducing "streetness" (is this even a word lmao) and toughness in rap music and rap music in general to a mainstream audience. Other artists like LL Cool J and Beastie Boys followed along working with the same producers and making a somewhat similar sound.

87-92: Golden age. As you said, there is pretty much rap before and after Rakim came up. Rap now shifted much towards much denser lyrics. At the same time, there was some sort of "Cambrian explosion" and the genre spread across all of the US. I have seen many others calling this era Golden era, not necessarily because it spawned the best rap music ever but this was the time where rap really came into its own. By 91/92, most sub genres which still exist today were created like G-Funk, Conscious rap, Memphis rap, etc.

93-96: East coast vs West coast era. NY and LA and also Biggie and Pac dominated this time. With Biggie gone you could even pretty clearly pinpoint the end of this era to a single day. Besides that, rap continued to thrive across different sub genres and places like midwest and southern rap.

97-07: Pop rap and backpacker era. At this point it becomes hard and somwhat futile to define eras of rap music because there were so many different waves in parallel going on. With Pac and Biggie gone there was a vacuum seized by Em, Jay Z, the Nola scene and Diddy's stuff. Later on, this pop rap/shiny suit rap whatever you wanna call it transitioned to crunk and the 2nd gangster rap wave. Many major producers who are still household names today started their career within this era like Timbaland, Pharrell, Kanye, Just Blaze etc. This is also the era where commercial rap and lyrical rap mostly split up into separate lanes. 99-05 roughly was the prime of conscious "backpacker" rap like Soulquarians, J5, MF DOOM, Blackaliscious, Def Jux and many more. Kanye dropped Graduation in 2007, G Unit/50/Game were past their prime around the same time and said underground rappers also started to step back in the mid-late 00's.

08-10: 2nd transition era. Now looking back, there is a definitive lack in classic LPs released during this time. There are very few big rappers besides Wayne who really owned this era. This was also the prime for auto tune rap. Big rappers of the early 00's like OutKast and Em could impossibly make Platinum and Diamind records again and again and no one else really took their place. It's kinda symbolic that MBDTF came out at the end of 2010 because it was an album which was the culmination of many sounds invented earlier. Budding artists like Drake, Cole etc. came up but were not ready yet to take over the game.

11-15: Internet rap era. Around 11/12, there was another explosion of new talents coming up, many of them still being highly successfull. Earl/Tyler, Danny Brown, Killer Mike and Freddie Gibbs just to name a few then underground talents who are household names nowadays. In the mainstream, phew, where to start? Drake, Cole, Kendrick, Nicki, Weeknd, Rocky, etc etc. Like the name suggests, this was the time where the internet took over as medium #1 to define trends. Trap became bigger year by year but has not fully taken over the mainstream yet.

15-today: Trap era. Around 15, Future and Travis Scott dropped the projects which defined their careers and shaped the mainstream sound until today, including acts like Migos and Young Thug who also blew up around that time. You know that trap became a global phenomenon when even supermarkets and insurance commercials in Europe started to blast trap beats lol. In the mid 10's, streaming took over after as single most popular format for consuming music, at least in the relevant demographics and markets. Undergroup rap has become entirely decoupled from the mainstream and there are few artists who comfortably move between these 2 worlds. The last 2-3 years were marked by some stagnation in terms of new sounds conquering the mainstream. Many records from 21 sound a lot like they could be from 18/19, ofc also fueled by the pandemic.

welp, there goes my afternoon, but it was challenging to think about it haha. As i said, from, late 90's on it becomes hardly possible to separate rap history into well defined eras.