What's the point of using open source build of VScode with all the proprietary Microsoft stuff removed just to later install the same the binary blobs manually? If you are fine with that might as well run the official Microsoft VSCode build the normal way.
Having some of the telemetry removed in VSCodium doesn't automatically remove it from the microsoft extensions. Taking into account that the official way for disabling telemetry in microsoft extensions depend on the telemetry setting in VSCode itself, having unsupported(and forbidden by microsoft license terms) combination of extension with vscodium seems like higher chance of disabling mechanism partially failing.
As for PlatformIO, luckily at least the project compilation and toolchain setup (which is the most important part of PlatformIO) can be done with commandline tool independently of any editor extension.
Having some of the telemetry removed in VSCodium doesn't automatically remove it from the microsoft extensions. Taking into account that the official way for disabling telemetry in microsoft extensions depend on the telemetry setting in VSCode itself, having unsupported(and forbidden by microsoft license terms) combination of extension with vscodium seems like higher chance of disabling mechanism partially failing.
As for PlatformIO, luckily at least the project compilation and toolchain setup (which is the most important part of PlatformIO) can be done with commandline tool independently of any editor extension.