I went through the same setup steps on another machine and managed to get things working successfully. The issue was that I had put the achievements under [OnlineSubsystem] versus [OnlineSubsystemSteam]
Here’s the important bit in DefaultEngine.ini:
[/Script/Engine.GameEngine]
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="/Script/OnlineSubsystemSteam.SteamNetDriver",DriverClassNameFallback="/Script/OnlineSubsystemUtils.IpNetDriver")
[OnlineSubsystem]
DefaultPlatformService=Steam
[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=480
SteamAppId=480
Achievement_0_Id="ACH_WIN_ONE_GAME"
Achievement_1_Id="ACH_WIN_100_GAMES"
Achievement_2_Id="ACH_TRAVEL_FAR_ACCUM"
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="OnlineSubsystemSteam.SteamNetConnection"