If anyone else has the issue of being stuck on 3G with poor signal after switching to a Pixel 6, I had to manually reset my preferred network type:
- Call
*#*#4636#*#*
- Select ‘Phone Information’
- Set preferred network to whatever you want but I chose NR/LTE/TDSCDMA/GSM/WCDMA
Voila, 4G works again.