Hello, If Asher or anyone may be interested, I have actually been continuing to work on a version that incorporates my own preference for the metric modulation formula, with the aim of including it in my own LilyPond Contemporary Notation Cookbook once it is sufficiently refined.
You may try the code from this link (as the code has become too long): https://github.com/yoshiakionishi/lilypond-snippets/blob/main/public-draft/MetricModulationFormula.ly Yoshi
