I would use the part, Microchip hasn't completely ruined Atmel's products.
The automotive industry uses 5V exclusively.
for I/O between sensors and ECU.
ARM parts such as NXP S32 Automotive are trying to dethrone the Power PC.
of course, those power cores (MPC5/SPC5) are approaching the end of the longevity program. And complexity/performance of chips and requirements have increased so much in so little time.
do you have any information that they will have 5V IO/s?
Anyway, i had some time with the C21, i tried i think three different MCUs and in the end settled with a PIC32MX for that project.
It took little time to understand the logic behind ASF. Datasheet was harder to understand than expected and it lacked a lot of information that may have been declared "obvious".
after a quick dive inside the various library files i was able to understand how the registers were actually named and that was it, no more ASF for me.
Or, if you still want to use a library, user ataradov has written a lightweight library for ATSAM and other ATMEL products (you can find it on his github). Altough not very hard to understand, for some things ASF was really convoluted, his library helped me understand better the inners of some peripherals.
the CAN controller in the C21 is okay, i guess. I had no major problems in configuring it and it worked okay in all the tests before requirements changed and i changed MCU as well