These discussions of "minimum viable population" sizes always seem to overlook a simple solution using technology that already exists and is mature and reliable: frozen zygote and gamete banks

That's not an oversight; you're talking about systems that are not designed for use or transportation outside of the magnetic field/ atmospheric protection of the Earth, much less the heliopause of our Sun. Currently, the three most viable solutions for dealing with the damaging effects of deep space radiation are currently:

A) Essentially replicate the magnetosphere of the Earth (sounds easy enough) by surrounding the ship in a giant artificial plasma shield, or

B) Surround the ship with mutliple meters of solid metal, or

C) Surrounding the spacecraft with thousands of years of human poop

One of humanities top idea's for dealing with the harshness of space is to turn ourselves into a giant turd. You're kidding yourself if you think the technology you describe is even remotely ready to endure the harshness of a galactic environment.