Expanding on a comment I made: manufacturers put surface coatings on their premium chains. Shimano and Campagnolo add low-friction coatings. SRAM adds what I think is a chrome-based treatment to their premium chains, i.e. the Force and Red AXS chains on the road, and (I believe; less familiar with MTB lineups) the Eagle XO1 and XX1 chains (compared to the GX chains).
Ben's statement about the Cyclingtips article did rely a lot on Adam Kerin's (Zero Friction Cycling) testing and experience. (Be aware that Cyclingtips got taken over by Outside Inc. Not all the data may display in the link, and it's also behind a metered paywall.) Kerin's current results are here. It is a bit more complicated than just more expensive chains don't last longer. For 11s Shimano chains, that did appear to be the case, i.e. Dura Ace had a shorter lifespan than Ultegra. However, the Cyclingtips article does show what I believe is Kerin's experimental data showing that the XO1 and XX1 chains (with hard chrome treatment) have much longer life than the GX chains (without hard chrome).
If you believe Kerin's testing is accurate, then you should consider the potential cost-benefit. Unfortunately, you won't know how much longer an XO1 chain would last with the current maintenance. The XO1 chain appears to have a street price in the US$60s, compared to the high US$30s for a GX chain. However, if the chain wears more slowly, this saves wear on the chainring and the cassette, and the latter is really expensive.
Personally, I believe Kerin's research is likely accurate, and it certainly appears very well thought through. Others must make their own assessments, naturally.
I don't MTB. The OP's stated 300km of life does sound very much lower than I'd expect, even if the GX chain isn't very durable. The OP's routine, however, would only remove dirt on the surface of the chain. This is not a criticism, in that most consumers probably do this. However, chain wear is accelerated by dirt mixing with lubricant and forming a grinding paste inside the chain, where the pins contact the rollers. If you can get this dirt out, it should improve chain life. An on-bike chain cleaner like this Park Tools unit would accomplish that job (many other manufacturers make their own versions). You would fill the tank with dilute degreaser, run the chain a few times, and preferably empty the tank and repeat the process a couple of times.
It would be even better to remove the chain and shake it with diluted degreaser in a bottle (e.g. old water bottle, scavenge a hard plastic drink bottle from the recycling). Again, this procedure is better if repeated a few times. People do use ultrasonic cleaners for this purpose, and they will work, but I find that shaking the chain in a bottle is more than adequate. The downside is that you have to remove the chain. SRAM quick links are officially rated as single-use. In practice, they can be re-used. However, SRAM and Shimano links snap hard by design when closed, and this puts a bunch of stress on the link. You would want to not reuse them once they're noticeably easier to close. You could buy links officially rated for re-use. For example, the YBN links should be compatible with GX, they are rated for 5 uses, and you may be able to get more uses than that. Wipperman doesn't make 12s links yet, but its quick links do not snap shut and can be re-used across the whole life of the chain.