There is also a 3rd style that I have seen in some bindings: refresh_Interval
Try each of the 3 to see which one works.
In theory, the json file should have a hint for this parameter (configure the thing and it’s refresh interval in PaperUI, then look at this parameter in the json file and then delete the thing and define it manually in the *.thing file)
REFRESH_IN_SEC is correct. Please be aware that the Yamaha Binding experienced some big updates in this development cycle. You may want to prevent the binding from updating otherwise your current text configuration will no longer work.
RELVOLUMECHANGE is used for the relative volume change factor, as you have figured out already. In the new version it is applied to a zone, not the AVR thing itself.