I think it's entirely possible that both Elflien and Lee stay, with Lee being the more likely to leave after the season. Lee hasn't had the season this year as he did last but I believe that is because of the position he played in the defense. He didn't attack the LOS as much this season instead playing more in coverage.
I agree mostly with your thoughts everywhere else. The defense will be good again next season with the biggest losses being Bosa, Bell and Perry. RB should be by committee but someone may well take charge of the position by mid season. WR will be interesting to see who steps up for Mike Thomas and in the slot as well. The way the young offensive line gels will be the telling factor.