Using PIO may be the “official” recommendation and I’m sure there will be users that can assist you with that should you wish to pursue that. But neither Robert or I (we helped you previously) will use PIO and both of us would recommend the Arduino IDE route rather than PIO, especially as you are not a frequent user and have experience with the latter method already. Plus your notes relate to previously using Arduino and may not be relevent to PIO.
If you use Arduino then you only need to focus on and accommodate changes in the actual firmware and IDE rather than also trying to add to that the learning curve of a new IDE eco-system. But the choice is of course yours.
I’m sure Robert will be able to advise on any progress regards a new continuous sampling FW (potentially in the pipeline, unless I missed the release) for the emonpi that would probably suit your application better than a shop emonpi due to you not having an RFM installed. However, again I would KISs to start with and just try the released latest version as per your initial intention until you are comfy with doing the task and know all the libs etc are in place before attempting to change to a significantly different FW that may not be release ready yet.
Dave is using a “homebrew” emonpi build and has previously had to compile and install his own FW due to NOT having a RFM installed. (and probably should have noted that in his post 
)