Something that always bothers me about these discussions is the absurdity of the premise. "What is the best rap mic under $5000."
Ok, that suggests that you've got some serious coin to spend, going for a high dollar piece of gear. It also suggests that you have some experience with recording and have at least a decent recording area and amount of additional equipment. Suggestions start coming in, $500 mics, $1000 mics, $4000 mics, $3200 compressors, $1000 and $2000 preamps,
As the discussion going on, it comes out that we're using a $125 interface/preamp with a $200 mic. What is the monitoring setup? What's the recording room or mixing room look like (or sound like)? We have yet to hear an example of what your voice sounds like, what you consider "flat and weak". Is it the mic, the setting, the monitors, the room, or the voice that is the issue here (not trying to trash your voice, but it is a MAJOR part of this equation)? There's not a mic made that will make Taylor Swift sound like Cher or Beyonce. And I'll bet you could line up 10 mics and record Taylor and EVERY ONE would be instantly recognizable as her.
Some pretty significant songs have been recorded on "less than pro" equipment, Billie Eilish with the AT 2020 is one example.
RadioWV recorded Oliver Anthony's Rich Men North of Richmond with an NT-1a and garnered over 150 million streams on Youtube alone. In fact, almost all of the RadioWV recordings have been done with an NT-1a, outside in somewhat remote locations. Lots of different artists, none of them really sound flat and weak.
Properly used, you should be able to get a pretty big fat sound out of the majority of mics out there these days. We are long past the days of cheap crystal mics, and $30 Radio Shack electrets for home recording.
I think we really need more to work with here, or else we can just tell you to buy a U87 or a ELAM251, a Neve 1073 preamp, a TubeTech CL1B, maybe a pair of Genelec or Dynaudio monitors, a stack of plugins, and you can stick them all in a crappy room, and still come out with crappy sound.