diff --git a/data_safe_haven/config/config.py b/data_safe_haven/config/config.py
index 543f49812e..b8cf991b10 100644
--- a/data_safe_haven/config/config.py
+++ b/data_safe_haven/config/config.py
@@ -8,17 +8,13 @@
 from azure.keyvault.keys import KeyVaultKey
 from pydantic import (
     BaseModel,
-    ConfigDict,
     Field,
-    FieldSerializationInfo,
     ValidationError,
-    field_serializer,
     field_validator,
 )
 from yaml import YAMLError
 
 from data_safe_haven import __version__
-from data_safe_haven.config.context_settings import Context
 from data_safe_haven.exceptions import (
     DataSafeHavenConfigError,
     DataSafeHavenParameterError,
@@ -43,6 +39,8 @@
     TimeZone,
 )
 
+from .context_settings import Context
+
 
 class ConfigSectionAzure(BaseModel, validate_assignment=True):
     admin_group_id: Guid = Field(..., exclude=True)
@@ -150,7 +148,6 @@ def update(
 
 
 class ConfigSectionSRE(BaseModel, validate_assignment=True):
-    model_config = ConfigDict(use_enum_values=True)
     databases: list[DatabaseSystem] = Field(..., default_factory=list[DatabaseSystem])
     data_provider_ip_addresses: list[IpAddress] = Field(
         ..., default_factory=list[IpAddress]
@@ -168,22 +165,13 @@ class ConfigSectionSRE(BaseModel, validate_assignment=True):
     @field_validator("databases")
     @classmethod
     def all_databases_must_be_unique(
-        cls, v: list[DatabaseSystem]
+        cls, v: list[DatabaseSystem | str]
     ) -> list[DatabaseSystem]:
-        if len(v) != len(set(v)):
+        v_ = [DatabaseSystem(d) for d in v]
+        if len(v_) != len(set(v_)):
             msg = "all databases must be unique"
             raise ValueError(msg)
-        return v
-
-    @field_serializer("software_packages")
-    def software_packages_serializer(
-        self,
-        packages: SoftwarePackageCategory | str,
-        info: FieldSerializationInfo,  # noqa: ARG002
-    ) -> str:
-        if isinstance(packages, str):
-            packages = SoftwarePackageCategory(packages)
-        return packages.value
+        return v_
 
     def update(
         self,
@@ -389,7 +377,7 @@ def from_yaml(cls, context: Context, config_yaml: str) -> Config:
             config_dict[section]["context"] = context
 
         try:
-            return Config.model_validate(config_dict)
+            return Config.model_validate(config_dict, strict=True)
         except ValidationError as exc:
             msg = f"Could not load configuration.\n{exc}"
             raise DataSafeHavenParameterError(msg) from exc
@@ -406,7 +394,7 @@ def from_remote(cls, context: Context) -> Config:
         return Config.from_yaml(context, config_yaml)
 
     def to_yaml(self) -> str:
-        return yaml.dump(self.model_dump(), indent=2)
+        return yaml.dump(self.model_dump(mode="json"), indent=2)
 
     def upload(self) -> None:
         """Upload config to Azure storage"""