I have been using OnePlus 5 for years now, mostly stock and just testing out DivestOS but the battery life is getting weak (recharge every 1.5 day on just wifi with minimal use on stock) and I simply would like an upgrade and more importantly to use GrapheneOS.

In Europe buying new Pixel is limited to 6 series, but I am looking USA eBay anyway, I don't have problem with used phones in decent condition. I bought used phones from eBay multiple times and I have no regrets. I am always getting unlocked one and in decent condition.

The 4 series is probably not wise choice by 2023 not only due to the battery age even if it can be replaced. And 5a seems obvious choice with great battery life. But with the offers I am seeing I can buy 6a for same price as 5a and 6 is $50 more and 6 Pro is another $50 or so.

Battery specs and GSMArena endurance tests are:

6 Pro 5003 mAh, 84 h endurance rating
6 4614 mAh, 86 h endurance rating
6a 4410 mAh, 94 h endurance rating
5 4080 mAh, 95 h endurance rating
5a 4680 mAh, 122 h endurance rating

I am not sure in which conditions these tests were made but GSMArena should have consistent technology methods for at least these 5 phones.

So 5a wins, probably not only because 60hz display, and 6 series is probably not yet as optimized as 7 and 7 Pro (those have 96h and 83h respectively) which are out of my price range.

My usage consist of wifi at home with slight usage for web reading (no video or audio), tasks, quick photo, weechat (irc) and some IM. Similar things when travelling, wifi mostly off, so LTE with sometimes location on and more photo taking maybe.

So how can GrapheneOS be optimized for longest battery life?
Provided that we would like to charge by night at ideally 20% lowest for battery voltage to not drop too low which should afaik help it's overall life. So also limit the charge to 85% or so perhaps.

As for features, I am 120Hz+ addict on PCs for years now and while my OnePlus 5 at 60hz is good enough, I seen Motorola G42 and Oneplus 7 Pro 90hz and it's nice, so 5, 6 and 6 Pro have a point there. Wireless charging I can't imagine yet but if one makes a habbit of it this could change things big time.

Then there is camera and few minor things I havent' researched in detail yet.