There has been some discussion regarding booking this ferry on the Lustrabaatane line, the smaller and classic, old-school boat described in the RS Norway guidebook. I believe they just opened their bookings for this time frame. There had previously been a concern that the bookings were already full, but I think they had not yet opened for the season. I have just booked for June 7th. Hope this information may be useful!
I recommend to change date at Flåm: https://www.flamport.no/arrival-list
the smaller and classic, old-school boat described in the RS Norway guidebook
Not sure if the non-electric ships still have permission to enter Nærøyfjord. See decision info of NMA.