there is a way using a short "routine +" formula to do just that and it works fine. I know it's not as clean as an actual setting in the ui, but it functions as you would wish.
Must download Good Lock and Routine + (for the Button function).
Download the Google Assistant app from the play store.
In (main phone) Settings, Advanced Features, Open Bixby Routines.
"Add routine" (at the bottom middle), Hit "IF" Scroll down to “Button Action”
Choose “Side” and “Touch and hold” Then “Functions” and “Open an app or do an app action”
Choose “Open Assistant”
Name the routine.