To keep this simple and clean, you know the basics drop a budget. Although there are a few side questions.
Would you be fine switching to Samsung if the price is fair?
Is image quality "really" a thing you bother with?
Screen quality something as well?
The LG G6 is alright, for the most part just an upped model of the previous generation phones. You can find them fairly cheap new and can be matched up to 2015 phones in relevance. You could also go to Motorola's 2015-17 model phones, and find a good selection for the price. The Turbo Two, which I still prefer over my S8/S7E is a good choice for the camera, and unshatterable screen, though it does have it's downside with quality from the screen. It is quite dim even under broad light, which is where I wish they would collab with Samsung.
The OnePlus 5 is one of the better models, I used the OnePlus 3t for a while and loved it. What I do really like about the OnePlus lineup is the internals, adequate RAM for really anything they overdo it each time, the CPU is alright, I mean heat machines in the term of use. Good phones, not the best with screens and well images are actually good.
If you opt for Samsung your best bets would be the S6/S6A or if you could find anything Note 4+ really is a great choice all around, the sound is a bit weird and took me a while to adjust to with them. I use the note 4 on occasion if I need a vivid picture of something. On the flip the S7E, if you could find it for a reasonable price used, or new, would be the second to the best choice for the £400-500.
You mentioned CPU/GPU well, in all honesty, anything 2015 upwards is fairly decent in terms of all-around performance. If it is really something you value, get anything from Samsung, or the OnePlus lineup prior from the 3t+.
You won't notice major differences, most phones suffice on only 3GB ram, and a Snapdragon/open-q 820 or higher. So that isn't much of an issue regarding performance, and if you aren't playing/doing highly 3/4D based tasks, you should be fine with anything listed above, for more "intensive and top of the line" you would be shuffling out over £500-800 which is pointless IMO.
I think Google did a thing with the Pixel on trade in's or something like that more recently.
On a flip note, you could make the G3 last longer it sounds like software fault or a shortage. If software and you are on a lower version of Android, you could flash a new or stock image, might help.