Sounds just like me. Nexus 6p, same problem with battery. My battery is complete crap. I have had the 6p since day 1. The OP 7 will be a day 1 purchase for me. The pro version while nice, some of it I don't care for.
I am still using my Nexus 6p from release date. I amwaiting for the OnePlus 7. I love my 6p but fingerprint scanner died today and my battery dies around 40%. I never did replace it.
I don't know why you get they are doing 64mb. The two options are 128mb and 256mb for the 6T and I don't see the 7 being below 128mb.
The 7 will be a day 1 phone for me. I am coming from the Nexus 6P bought it day 1. Having never used a Oneplus phone before my only things I would like...