BRN-10, Jackyl, CMMG Dissent all rely on a buffer tube ring and will crack the back of a printed AR lower.

https://hoffmantactical.com/designs/sl-15/

This will happily cycle an AR 10 upper with a buffer tube.

You have a good idea about reinforcement. Screw the endstop block into a blind hole in the back and have a monolithic lower with stock. Like a short stroke WWSD rifle.