MS is actively making your life using VS Codium a pain. They removed the download button the extension marketrplace making it very difficult to download extensions and installing them in VS Codium since VS Codium does not have access to the official MS extension marketplace. Many don't publish outside the marketplace for example Platformio. [1]
1. You can add a bookmark that executes enough JavaScript to download the VSIX as usual.
2. I think you can patch the product.json from VSCodium to use VSCode. Gets overwritten on every update probably.
Honestly though, it's easier to disable ~three settings in VSCode and call it a day.
I've not struggled to find the things I need at https://open-vsx.org (usually by searching directly within VSCodium), but then I only use it for editing things like markdown docs and presentations, LaTeX/Typst, rather than coding, which I prefer to do in a terminal and with a modal editor.
[1] https://github.com/platformio/platformio-vscode-ide/issues/1...