There indeed seems to be a limit to the message rate you can get using rtl-sdr compatible dongles.
Probably messages are overlapping but i’m not exactly sure about what the problem is.
But with an external LNA and a good dongle (the rtl-sdr blog v3 seems pretty good) at least I could reach message rates up to 1450.
To recover even more messages you’ll need an airspy or a ModeS Beast.
I’m not quite sure why they are able to pick up more messages though.
Another solution would be to sectorize your receiver.
2 antennas which cover 180 degress field of view each connected to their own USB receiver. (not really sure how to construct something like this)
If you want to run FA mlat both receivers will have to be on different stations.
Looking at your feed, the number of positions at <40 nm seems very low.
Maybe you should reduce your gain (Thoughts on optimizing gain)