I mean, you got it for a nice price. If the backlight bleed isn't as bothersome in real-world use as it is in the pictures, keeping it wouldn't be a bad decision if you like the physical dimensions and screen resolution. You'll find it very difficult to find another brand new 3440x1440 monitor at that price.
Also, consider what brightness setting represents typical use for you. If your pictures are fairly close to what you see in real life, that LG backlight is so excessively bright that I would have the brightness turned down even lower than 30. This is my Acer X34 at 30 brightness and it's pretty much what I see in real life:
http://www.overclock.net/g/i/3054328/a/1636965/acer-predator-x34-owners-club/sort/display_order/
If you do decide to return, since you're in Europe, I can't say for sure whether Amazon's return policy is the same as it is in the US. Either way, I would consider excessive backlight bleed a defect that should be just cause for them to provide prepaid return shipping.
When I was shopping a few months ago, I was primarily looking at adaptive sync 34-35" 3440x1440 monitors. For 2560x1440, I only looked at 32" screens because I wanted to fill all my remaining desk space and 27" was too small. At the time, my only 32" 2560x1440 option was the HP Omen 32, which didn't excite me. For 3440x1440, I looked at the LG 34UM88(C)-P, LG 34UC88, AOC Agon AG352UCG, Acer Predator X34, and Asus PG348Q. I ended up with a factory recertified Acer X34 for $670 or 575€, which made me feel a lot better about the monitor not being perfect, because I was not going to pay the regular $1100+ retail price.
I'm not the best one to seek advice from for high-refresh FHD monitors. Since your budget will definitely allow for 2560x1440, I would go for the extra pixels. While I don't think FHD will become obsolete in the very near future, support for higher resolutions will always be welcome going forward.
One other thing to consider is how you plan on using the monitor. Gaming at 3440x1440 at mid-high settings while staying comfortably above minimum adaptive sync range in the most graphically intense games will require at least a GTX 980 Ti or 1070, or an RX Vega if you're looking for AMD/FreeSync compatibility. Otherwise, 2560x1440 or 2560x1080 (21:9) will be more forgiving with video cards below the 980 Ti level for mid-high detail in the latest games. For any other purpose including less graphically intense games, 3440x1440 is great with most recent video cards.