You should be able to use other chips in the EFM32 family as a drop in replacement, obviously requires soldering your own board. Seem to be available with up to 128K flash and 16K RAM.

Of course the GnuK site lists some STM32 boards designed specifically to run GnuK. Or if you don't care about open hardware you can buy a $2 STLink clone on aliexpress and flash GnuK on it.