You are correct, the geometry method & some other specific settings are saved with the favourite for 2D Lines. I'm not convinced the other unique settings are necessary, but I can see how that may be a problem for some users if they were removed. However, if you look at the dimension and label settings you can pick a Text Style favourite to be used, perhaps there is scope to use the Line Tool favourites as a similar sub-setting for circles, splines & polylines?
It does raise the question though, why doesn't the text style favourite selection auto update if the source favourite is changed? I would hope it would for line styles.
I certainly don't see a way to merge the slab, roof & shell settings into a Favourite just now as there are a lot more saved variables in play.
Bottom line I think is the favourites only display the available options for an associated tool so there is no reason why we shouldn't be able to set identical favourite names for consistency across similar tools apart from OS file management if that is really an issue.
Apple iMac macOS Ventura / AC26UKI (most recent builds)