What resources are there on tone system creation? Resources could include
- Documentation on conlangs that have tone systems
- Guides on tone system creation
- Resources on tone in natural languages, for a posteriori naturalistic conlang creation
Artifexian has a decent pair of videos (#1, #2) on tones which discusses various types of tone systems, how they work in natural languages, how they developed there, and ways you could work them in to a diachronic approach to conlanging. I don't think it discusses conlangs that do have tone systems though.
The Wikipedia page for Four Tones (Middle Chinese) also does a decent job of discussing the evolution of tone across the Sinitic languages.