sparkfun is the single source supplier (and now maker of the product).
Spinning an IMXRT1062/IMXRT1064 design sans the terrible Teensy bootloader should take a day or two at most.
These chips have perfectly-fine ROM USB bootloaders and SWD, don't ruin them by adding extra garbage.
Yes, obviously, but they don't make the chips, so can't you just source the exact same chip, make thing pin compatible and call it a day? Then you'd have a drop in replacement, any changes you make will cause disruption for people downstream.
https://www.nxp.com/part/MIMXRT1062DVL6A