Force a single value base field to be in a separate table?
Seems the following class is responsible for determining a separate field storage table:
\Drupal\Core\Entity\Sql\DefaultTableMapping::allowsSharedTableStorage

It returns a boolean based on the following 4 conditions:
public function allowsSharedTableStorage(FieldStorageDefinitionInterface $storage_definition) {
  return !$storage_definition-&gt;hasCustomStorage() &amp;&amp; $storage_definition-&gt;isBaseField() &amp;&amp; !$storage_definition-&gt;isMultiple() &amp;&amp; !$storage_definition-&gt;isDeleted();
}

The only feasible solution would be to implement your own Storage handler and flag the basefield with &quot;custom_storage&quot; =&gt; TRUE