I am not a scala fan and do not care for it, but I upvote for the thorough thought process, breakdown, and debugging of the problem. This is how technical blogs should be written. AI aint got shit on this.
If you're going to "refresh" a codebase you probably want it to be on the current version of things. Old dependencies rot, like it or not. I don't think there's any timeframe for Scala 2 EOL yet, but new development is happening in 3.