Sorry for the delay - I missed this post...
Strange about the nRF52840/nRF52832 being different - as you noted the 532 generally works pretty reliably...
The issue has been as far as I know that when you enable UARTE mode the UART DMA hardware doesn't actually interrupt you when a byte is received - it only interrupts when the buffer you give it is full - which is the reason why the library uses 1 byte DMA buffers (which is worse than not using DMA - I did try!).
So perhaps the library mentioned in the article that @fanoush mentioned above might work - but we'd need a special library that is able to hack around the UART hardware - there is some hint of timeouts being used there, which may work but may impact power usage significantly.
Right now this is low priority for me though - nothing I sell uses higher speed hardware serial on nRF52840, and while I do provide builds (for some reason) for the RAK5010, I haven't been paid for doing so in 3 years so I'm definitely not doing free support :)
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