You lost me a little bit.
John Franklin-Myers and Sebastian Joseph-Day were both drafted in 2018.
Franklin-Myers played in all 16 games, and was the top-reserve-DL behind Donald, Suh and Brockers
Joseph-Day was on the 53-man-roster but was not a game-day-active. Morgan Fox was on IR.
Suh's one-year-contract expired; and Joseph-Day became the third starting DL. He didn't replace Brockers. Joseph-Day replaced Suh. Fox and Gaines were the top reserves in 2019 ... and again2020, behind the same three starters.
The Rams decision to release Franklin-Myers in their final cuts in 2019 was a surprise.
I think I agree with the rest of your post.