There have been previous feature requests for this very feature of start-up volume settings.
The other posts are old and felt a new posting would help.
I occasionally use Roon to play via Airplay to endpoints such as Sonos and HomePods in a group. House party mode!
Mainly on Sonos, the devices will automatically get set to 25 on the Roon slide scale. I do have a limit on top volume via the Sonos S2 app, but neither Sonos or Roon apps have a start-up volume. Sonos does have a start-up volume for line-in devices on the Fives.
My only work-around is using Ropieee XL or similar for airplay via Line in. Then set groups in Sonos and then (I think, not tried) enable the Sonos group in Roon. Not ideal or smooth.
Simply needed is a auto start-up volume limit inside Roon. This’ll solve all requests for such feature for all equipment makes