Failed a second time, apparently it now works _without_ that option set.
It used to be it'd fail to build unless I did set the "max old space size". This is a VM with 8GB of RAM… it did get an upgrade when the Atom C2000 issue was resolved on a couple of old boards meant I gained some extra compute capacity… but yeah… JavaScript drives me nuts.
Anyway, managed to get the assets compiled… onward with database migration.