You’d probably be better off asking on XDA under your specific device’s subforum.
I haven’t rooted or installed a ROM on any devices in like 6 years, but it was possible to install a ROM just using fastboot to flash the partitions, it isn’t as easy since it takes multiple command strings. It should still be possible since all TWRP does is flash partitions. You’ll just need to flash each partition manually or write a shell script to save you the typing.