I finally got this fixed. After looking at the logs, I found that the INVALIDARGUMENT error was due to entries in the mongod.conf file. One was a journal entry and the other was enableMajorityReadConcern, both of which are no longer valid in 8.0.
So, I removed the 2 Pi 5 devices from the replication set and then proceeded to update the PC from 4.4 though to 8.0 via 5.0, 6.0 and 7.0 updating the compatibility version as I went along.
I then deleted the database files on the Pi 5 devices, added them back into the replication set and started up mongod expecting them to synchronise. However, I have a corrupt document somewhere, so the replication failed. I overcame this by copying the database files from the PC to the Pi 5 devices. Started up mongod on the Pi 5 devices and all worked.
I still have a corrupt document. I don’t know how to fix that, but I can live with it for now.