As already mentioned above the lastest changes in flexcan driver sound promising.
One commit note:
make use of rx-offload's irq_offload_fifo:
"This patch converts the flexcan driver to make use of the rx-offload can_rx_offload_irq_offload_fifo() helper function. The idea is to read the CAN frames already in the interrupt context, as the depth of the flexcan HW FIFO is too shallow, resulting in too many missed frames. During a normal NAPI poll the frames are the pushed into the upper layers."
The statements of https://community.nxp.com/thread/487216 go into the same direction.
Unfortunately, there are too many changes from 4.9 to 4.19. So, a simple patch is not possible.