This BepInEx plugin adds inventory sorting buttons to Valheim. The player inventory is sorted excluding the hotbar, containers are sorted entirely. There is an auto-stacking feature which can be disabled via the configuration file (it's the only option in the file.)
Warning: There are windows-specific post-build events to create a zip file for uploading to https://valheim.thunderstore.io/ in the Visual Studio project.
Notice: I don't have much free time so updates to this plugin may take a while, and it may take a while before I notice new issues.