There's also https://github.com/JereKuusela/valheim-upgrade_world
Worked mostly fine in adding everything to >2 yrs old map that was added to the game in between (even to already discovered/generated areas): onion seeds to mountain chests, tarpits, FrostedCaves, Mistlands, etc.

Only got some janky behavior with FrosteCaves when I ran the commands for Mistlands (I think).