You may have the GPU drivers installed but are they active? Look in "Software & Updates" on the Additional Drivers tab and see which drivers are active.
Installing the drivers is not enough, you have to select them to use them too.
If the latest drivers are active then you may need to think about switching to a legacy version (you have a pretty old CPU and GPU by current standards; newest drivers are not always best). You may also want to look at using older versions of Proton than the latest for similar reasons - there may be features and changes in newer versions that are just not going to work with your set up or your set up just isn't tested to work with.