Premature chain wear GX Eagle After 300 km GX Eagle chain is at 0.5% wear which indicates to me I have to buy a new chain?
Is this normal? How to prevent it or advice for better chains?
I mostly drive MTB in the woods in Belgium in a dunes area. So a lot of very short and steep climbs and descents (think 50/100m). My gear ratio on a 34 tooth chainring is mostly on the 3 smallest and on climbs on the 3 biggest. I am running SRAM Eagle SX derailleur, with NX cassette and GX chain.
My bike gets washed almost every ride with me also cleaning the chain, cassette, sprockets and chainring from dirt. If I don't wash the bike it's because it's not dirty.
How I wash the drivetrain:

use garden hose to sprinkle of the biggest dirt from chain
clean chainring and sprockets with toothbrush
clean cassette with larger brush
use 2 nail brushes and let chain run in between
use garden house to get remaining dirt of
use an old rag and clean/dry chainring and sprockets
use old rag and thoroughly rub chain back and forth to dry it.
lube chain with a drop on each roller
go through a few gear high and low to let lube sink in.

Lubing with is done with wet lube since there is a lot of rain lately.
Any advice on better more lasting chains or does somebody have a similar experience and a possible solution?
Yes.  Get a new chain.  When I was running Sram Eagle, I found my chain worn out pretty early too, and I also found it made a big difference to change earlier rather than later.
In my experience, this is within the range of normal...on the low side of normal for sure, but not anything that I think you are doing wrong.
Keeping your chain clean is a good thing but not fool proof.
Lastly, I remember reading an article several months ago where several chains were put to the &quot;wear test&quot;.  The conclusion was that more expensive chains don't last longer.  In fact someone at a bike shop told me once that the more expensive chains actually wear quicker because they are made with weight in mind.  I don't know if that is true or not.   If I can find that article, I'll add it to the post.
Good luck.