Hi @Stuart P
Yes your assumption seems correct because SYSVOL replication , is managed in domain level not forest level and sysvol settings are saved in domain partition.
For you case , migrating FRS to DFSR for sysvol replication in child domain, let you promote the first domain controller under Windows 2019 in child domain if the domain and domain functionel level is Windows 2008 R2 or higher.
In other hand , I recommend you to migrate the sysvol replication for root domain to DFSR asap because FRS is no longuer supported by microsoft.
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