Thank you for the writeup! We do not have any near term plans to offer additional variable backed configuration options for the XY Chart widget.
For now, function backed chart layers are a catch all for dynamic chart configuration.
We are more likely to invest in the Vega chart widget in the future due to the flexibility it offers. The XY Chart will remain available so if you wish to use it investing in function backed series is worthwhile over waiting for more variable backed config options.
I answered a similar ask for more variable backed config here highlighting that adding variable backed config is not always trivial since it requires us to handle async loading states for values that are currently always statically available.