You are not the only one having this problem. I'm shocked it's been an issue for so long! I just got the s10e and new smartwatch [upgraded from gear s2] and I'm having the exact issue. Its NUTS.
Sorry to say I haven't found any reasonable solution.
New 46mm galaxy wearable user and I have to turn my watch off to use my handsfree ? found this thread and wondered if any advance had been made with this issue as I have some 20 days to return it if not, as there is no point not being able to use my handsfree in the car whilst wearing the watch ??
I have looked at a few apps that say they proritise bluetooth connections but non work with the galaxy, so im going to change for another watch unless I can resolve this, which is a shame
Due to forum problems (I guess with long threads that gives the "unable to ..") I answer "Stubbie1960"
with a follow up to the first post. Hope this is OK.
Anyway, the workaround I use to let my phone automatically connect to my car (bluetooth handsfree) I enter Bluetooth settings in my phone, open settings for the Galaxy Watch and disable the call audio for the watch. (I can live without taking calls through the watch but in my car handsfree is essential.)