i was in same situation like you,,start RM6 only on fastboot but thie works ,download you rom from this page https://drive.google.com/drive/mobile/folders/1BE4FSGDcfXavYoJ3Kw3toGMiPHgCGakC?usp=sharing
extract boot.img to your folder with fastboot
start mobile in fastboot
and open cmd.
if you see your device try next command
fastboot flash boot boot.img
restart your phone and you have stock recovery,,wipe data and you have unbricked device
i download latest twrp but not working and after install twrp i was stucked in fast boot like you