Another one is switching to another language when their first foreign language is not, at least, B2 level.

This one is painful for me to read on multiple levels. First, one language learned well is worth 5 learned to a lower level [defined as you don't know the languages well enough to consume native media with ease]: it's easier to maintain and it's much more fun to use.

Second, learning one language independently all the way through gives you crucial insights into the learning process that are often hard to see unless you've experienced them personally. A good third of the questions in this sub would be eliminated if learners learned one language, independently, to a solid C2 before branching out.