It is of course a gradient, however, on that gradient there exists an important threshold: can this person, under their own direction, produce positive net value?
Juniors require your active guidance. Seniors require your availability.
Juniors require your active guidance. Seniors require your availability.
The one you propose is useful and how the literature on expertise defines a journeyman but the word "senior" is highly ambiguous.
Given context, an otherwise ambiguous word can be useful. "Senior" needs a lot of context, though.