It is indeed, it only takes about 5 minutes to build! I added the stuff the gentoo wiki says is required for Luks, let me know if it is what is required or if you need some other stuff in there as well. As far as your USB issues go, are you using a powered external enclosure for your SSD? If you are not, that is most certainly the root cause of the issue. The Pi powersupply won't give full power to video and all usb devices if you have too much drawing power, it isn't a super hefty psu, you may want to consider getting a good USB-C type card that has usb, and it's own powersupply. Although if you are not currently using a powered drive enclosure, that should solve your issue. I also recall reading somewhere you can use the gpio pins to add power, may also be something to look into.
https://www.dropbox.com/s/2adrmtna5p7nx ... ar.gz?dl=0
There is also the newer uboot on dropbox here, but it is bundled with an older kernel.
https://www.dropbox.com/s/u2a4f5d9dawn2 ... ar.gz?dl=0
Good luck, and let me know how it goes for you.