You can't get great camera, performance, build quality and features for 300€. For that price something always has to give.
In that range I would look at a handful of phones. If you want a small screen the Nokia 8 is probably the only valid option.
If we're talking around 6" I'd look at four phones in particular.
Honor Play: Huge average screen, good battery life and performance in a nice and solid aluminium body. Decent camera. But overall a typical emui phone that is overall very good value for money.
Nokia 7 Plus: 5.9" with a very good panel but a lower screen to body ratio than the competitors. Good battery life, solid but not exceptional performance, very good full body aluminium construction. Unparalleled build quality in this price range. Android One with all its advantages like rapid and regular updates. The camera is decent and it has a second one with 2z optical zoom which is a very nice feature to have.
Xiaomi Pocophone: if you really game a lot and focus on raw computing power than the Pocophone is an incredible value proposition and beats the phones mentioned above. It is a full plastic build though, the very wide notch is imo ugly, the display is average, it has no NFC and I know that the phone has annoying issues with its notch notifications. The camera is decent. Still, you get a whole lot of performance for your buck.
None of the phones above are exceptional in both the performance and camera departments. At that price point something always has to give.
So another phone you should consider is last year's flagship from Xiaomi. The 128/6GB version of the Mi 8 can be found on sale for as little as 340€. Its camera is on a completely different realm of quality compared to all the other phones mentioned here.
It combines Pocophone performance with a great camera and nice materials. If you can afford and a decent to good camera is not enough for you then it I think it is very much worth the additional investment.
Lastly one might wait for the release of the XIAOMI Redmi Note 7 Pro. However I am not fond of cheaply constructed glass and plastic sandwiches which might break if you accidentally sit on them at an unfortunate angle. However there's plenty of information on it out there as well. I personally think it is still too expensive for the offered build quality.