Very good points.
I think it all just boils down to time and practice.
Watch Roberto Blakes first few videos, his delivery was very flat and monotone. Doug had a LOT of dead air between sentences. We all are always learning to evolve our delivery over time. The more you do it, the better you get.
I personally think the best way to vocally deliver, is to remember that natural speech doesn't follow patterns. Some people end a sentence on a higher note which makes it sound like they're always asking a question where as ending a sentence on a down note indicates the end of a point, and ending on a mid note indicates a point is going to be continued.
I'm learning that the note you start a point on can also impact how it is delivered. Sometimes starting a sentence on a high note can give a different energy than staring a sentence on a low note.
After doing voice overs for a while now, I've noticed there's one YouTuber, and Jamie Oliver suffers from this sometimes as well when delivering sentences, he starts on a low note, mid sentence goes high, and ends on a down note. But because he does EACH SENTENCE THE SAME... it starts giving off too much of a scripted unnatural tone of voice which can throw some people off.
I try to be very mindful of this, so I don't deliver all my sentences with the same flow. I'll mix up my flows to stop sounding too scripted, because our natural speech flows in irregular flows, not repetitive patterns.
This is definitely something i'm studying more of.
Nick Nimmin mentions this channel, Charisma On Command time and time again
https://www.youtube.com/user/charismaoncommand their channel may help more.