while Pierre’s solution works with some adaptation I think your approach is
Tempo is something that is created at the Score level, while TextScripts are
created at the Staff level. This means that it should be expected for
TextScripts to be closer to the Staff. Rather use the correct devices for what
is not a staff text, but in fact a label for a part or section. So rather use a
SectionLabel (which also lives at the Score level), and style it to look the
way you like it to look.
Half as fast
is in fact a Tempo mark, so include it into your tempo mark. A tempo mark can
use multiple lines by using \column.
which I’d argue is cleaner in both code and implementation.
Many thanks both to Pierre and to Valentin for their solutions!
All the best,
(he, him, his)