OK, firstly, yes, this is a suitable place to propose tab synonyms. You could also propose them directly from the https://ux.stackexchange.com/admin/create-tag-synonym page, but posting in Meta is useful as it's possible more people will see it (we don't get many synonyms on UX.SE so people don't tend to go and review the existing suggestions)
There's more info on synonyms on the main Meta.StackOverflow site: What are tag synonyms and merged tags? How do they work?
Anyway, onto your main question:
I don't think that ergonomics and physical are a 1:1 match, so don't really fit with being synonymed in my opinion. Yes, many questions tagged ergonomics are likely to also be tagged physical but I don't think it works the other way around.
Not every physical question revolves around the ergonomics of said physical item; it could be concerned with the visual / symbolic aspects of the item, the audio noises made etc. Tagging for ergonomics would (in my opinion) only be done if it's an object you physically interact with, such as a door handle, but not for questions about more general uses of the object, such as the whole door itself.