Careful, this may be easier said than done.
Each file in the "amix_2.1_tape_part1.tar.bz2" and "amix_2.1_tape_part2.tar.bz2" archives must be copied to the new tape in exactly the order given by the respective file names (00..28; ignore the "seglist" file).
If I remember correctly (I last set up my A3000UX back in 1996, using an Archive Viper tape drive and a hacked installation disk), the new tape has to be tensioned and rewound first before you begin (use the "mt" command).
After that, you write the individual files to the tape drive using the Unix "dd" command.
Make sure that when you write to the tape device the tape will not be rewound after each invocation of the "dd" command. Back in the 1990'ies NetBSD had separate tape devices for rewinding and non-rewinding access. Other operating systems were using non-rewinding tape access by default and required the use of the "mt" shell command if you wanted to rewind it.
Finally, make sure that your tape drive is accepted by the installation disk. While you may be able to knit your own Amiga Unix installation tape using the tape drive of your choice, the stock installation disk only works with a specific Caliper drive (rebranded by Commodore as the A3070, if memory serves). It won't work with a DAT streamer or any 150 MB QIC drive of the same age, with only 1-2 exceptions.
