Strangely, after all this time I'm finally trying my hand at xml modding, but something seems to be wrong with S3PE that I can't figure out.

When I open a copy of gameplay.package, it only shows 4 lines with the tag S3SA. The preview shows -

Version: 0x02
GameVersion: 0.2.0.209
Unknown2: 0x2BC4F79F

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RuntimeAssembly: Assembly
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Sims3StoreObjects, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
CodeBase: file:///F:/Game Stuff/Sims3 stuff/s3pe/s3pi.ScriptResource.dll
EscapedCodeBase: file:///F:/Game%20Stuff/Sims3%20stuff/s3pe/s3pi.ScriptResource.dll
FullName: Sims3StoreObjects, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
EntryPoint:
ExportedTypes: Error reading Value: Exception has been thrown by the target of an invocation.
DefinedTypes: Error reading Value: Exception has been thrown by the target of an invocation.
Evidence: System.Security.Policy.Evidence
PermissionSet: <PermissionSet class="System.Security.PermissionSet"
version="1"
Unrestricted="true"/>

IsFullyTrusted: True
SecurityRuleSet: Level1
ManifestModule: Sims3StoreObjects.dll
CustomAttributes: Error reading Value: Exception has been thrown by the target of an invocation.
ReflectionOnly: False
Modules: System.Reflection.RuntimeModule[]
Location:
ImageRuntimeVersion: v2.0.50727
GlobalAssemblyCache: False
HostContext: 0
IsDynamic: False

References:
Ref: Sims3GameplayObjects, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Ref: Sims3GameplaySystems, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Ref: SimIFace, Version=1.0.0.50, Culture=neutral, PublicKeyToken=null
Ref: mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Ref: UI, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Ref: System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

Exported Types:
Cannot get Exported Types: Could not load file or assembly 'Sims3GameplayObjects, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.
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I'm using S3PE version 17-0520-1821, and .net 4.8.1

Any ideas on what I'm doing wrong would be greatly appreciated.

gameplay.package contains some of the DLL files for the game, whereas GameplayData.package has the XML files I assume you're looking to modify. It can be found in Gameplay inside the bin folder