For what it's worth, I've always left Signal on the default battery profile (Optimized). It starts at boot as expected, and reliably delivers messages in real-time via WebSockets throughout the day. I believe it's the Alarms and reminders special permission that allows this to work as intended. The same goes for K-9 Mail with push enabled.
The only apps of mine that have required unrestricted battery optimization are DAVx⁵ and Syncthing.
Still, I agree that it is WebSockets that is likely draining the battery a bit quicker than Play would, regardless of SMS functionality.