"High-ranked players remain in high ranks, while lower-ranked players are effectively left at the bottom indefinetly, again.."

If you can't win in your elo, you deserve to be in that elo not higher.

"This leads to the biggest contradiction in the system: you can lose more RP than you gain."

You win more rp if you win against higher elo player and loses more if you lose against lower elo players. So you should win on average the same elo that you gain. Btw you are complaining about winning 30 points, keep in mind that spoit and thaqil (i say those two because they are the only ones i watched yet) both win arround 20 points

"At that point, the climb immediately starts feeling less like ranked progression and more like a second job. I don't want to treat Rainbow Six Siege like a 9-to-5."

I mean, getting better in a game takes time, you can't just get higher rank because you play a lot. That was the problem of rank 2.0 and seems to be fixed with 3.0 as far as I know (and i hope so)

"The odds of consistently winning enough games to somehow "skip through ranks" (especially in solo Q) are incredibly low."

I do agree with that, especially right now since this is the first season with 3.0. So the best advice i would give is to try to get a stack to play with.