Ok lets get serious. it could be a corrupt iOS installation, This will diagnose it.
-Unpair and erase the Watch (a) - see resources below.
-Backup the iphone (b)
-Restore iphone to factory settings (c). You should end up with the Hello screen on the iphone.
-Set up the iphone as new without restoring the backup. (d)
-Install no extra software, no social media, nothing else. (aka "vanilla minimal")
-Pair the Watch to the iphone (e)
Try Siri again
Only when that test is over can you repeat the three actions (a) to unpair, (c) to wipe then (d) to set up the phone but restoring the backup this time. then (e) to pair the Watch again
Scenario 1. If Siri works (vanilla minimal) then it proves it is software on the iPhone causing it.
In that case you have two choices.
1.You could export as much data as possible such as un-sync’d photos and documents out of the phone to a place of safety such as iCloud before repeating the restore and set up as new process, (a) (c) (d) (e) then build up the phone again from scratch. Obviously no security or VPN software.
Or
2.You could systematically uninstall software and any profiles until the battery will last a full day,
Scenario 2. If Siri works after both setups - ie with minimal software and with the restored backup then it suggests it was a simple corrupt iOS installation. You have fixed it - great news!
Scenario 3. Siri doesnt work in either state.
No idea at all .
Resources you need: (Allow a couple of hours for each session)
(a) Unpair and erase your Apple Watch – Apple Support (UK)
(b) How to back up your iPhone, iPad, and iPod touch - Apple Support
(c) Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(d) Set up your iPhone or iPad - Apple Support
(e) Set up your Apple Watch – Apple Support (UK)