Ouch… that may not be possible. PlatformIO depends on the Microsoft C++ extension, which doesn’t look like it will install on VSCodium as is… (meaning reversion to the Microsoft Marketplace is needed) not even the ~20MB ‘offline’ package from Releases · microsoft/vscode-cpptools · GitHub. If that installed, the PlatformIO VSIX would probably have installed fine… but as you say, it doesn’t work… just sits there for a minute, and then does nothing. 
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