Before you make any changes select all the clips in the thumbnail timeline in the Color page. After you change he settings, there is a button on bottom left of the palette which lets you apply the changes to all the clips.
Can you please make sure you have write access in the folder configured for render cache. It’s under the first configured volume in Media Storage in Preferences.
Select multiple clip thumbnails on color page. Change the settings you want. then press the “Use Changes” to “Use Settings” to either apply just the changes, or copy the settings to all the clips.
If you have a modern GPU and reasonably fast disks it’s best to stay with native files. If you are not getting Realtime playback, then you should transcode to DNxHR or ProRes.