I have broken apart the superroute so instead of >18k ways, it now contains six relations (one for each country).

Membership #s are:
Spain 6771 members
France 6103 members
Italy 3444 members
Slovenia 832 members
Serbia 539 members
Hungary 1244 members.

So, While Slovenia an Serbia are OK and Hungary might be "maxed out/borderline," all of Italy and especially Spain and France will need to be still further broken apart.  Let's say to <2000 members each, as a next step.

I may have made some errors, but this is a nudge in the right direction (smaller relations that are more properly constructed to OSM standards).  The job is not finished and should be continued by European mappers.