Hello Jeroen,
The limitation you've encountered is real, the standard "Related Record" field in SF VoiceCall objects cannot be directly mapped from Genesys participant attributes set via data actions in Architect flows. Your solution of using custom fields and a SF record-triggered flow is the recommended workaround.
I would love to see some community members speak about for more of a solution though!
Cheers,
------------------------------
Cameron
Online Community Manager/Moderator
------------------------------