The whole PBN thing may turn into a little farce due to the recent amendment of Part FCL (2020/739). The only time you are going to have to do PBN is on an initial Instrument Rating. Thereafter there's a new derogation which reads:
No more RNP APCH?
So if you haven't got the kit in your helicopter why bother with the whole thing in the first place! The PBN privileges that are left when you remove RNP APCH are minimal!
Plus they don't mention how you will annotate this reduction in privileges on a licence.