I'm currently looking at making some optical adjustments to a display font in order for it to work for titling. It's a bold no-contrast sans serif, with three masters and 8 weights from Thin to Ultra Bold. The variation in weight provides a good starting point for the optical size, and most glyphs don't actually need to change because the counters are already quite open and there are no features to dress down. I've started to add the few that need changing as alternates (maybe 4/5, mostly making them wider and lower the crotches where needed). However, the spacing would need to be different, much looser. The default spacing is currently super tight (on purpose) so tracking works to a point but it would be better if I could provide a different set of metrics.
What are my options? Creating an optical axis and duplicating 90% of the font (and having to change my brace tricks to bracket tricks)? Duplicating the font and have two font files?
I realise this is probably a slightly unorthodox case so happy to take unorthodox answers if needed.