Young RHDs with legitimate top-four potential almost never reach free agency, and they're rarely available via trade. When they do become available, you're usually paying a premium because every team in the league is looking for the same thing.
Even if you target an RFA or a young player through a trade, you're likely facing both an overpayment and the risk that the player never develops into what you're projecting.
That's why I don't see adding someone like Carlsson or Andersson as being in conflict with drafting and developing a long-term RHD solution.
In fact, I think they're complementary strategies.
Boston could sign a veteran stopgap, draft one or more RHDs with some of the premium picks they have over the next three years, and continue developing the position internally. The reality is that even if you draft the right player tomorrow, he's probably not helping your NHL roster in a meaningful role for another four or five years.
That's why I view Carlsson or Andersson as solutions for one timeline and a drafted RHD as a solution for a different timeline. Those ideas aren't mutually exclusive.