For out-of-tree build, use KEFIR_BIN_DIR make parameter. I use it quite extensively in https://git.sr.ht/~jprotopopov/kefir/tree/master/item/script... so this should work.
DESTDIR is supported as a parameter of make install: https://git.sr.ht/~jprotopopov/kefir/tree/master/item/instal...
As for the parallel build issues, contact me over the email please