No software on a modern computer *(any modern computer) will be able to read what Retrospect puts on a tape.
Your best bet for "backup" is, if you aren't going to build a network, to buy a SCSI2SD or an AztekMonster and copy your data there. From that point, you can actually take the original hdd out of the machine and then boot from the scsi2sd or aztekmonster. You can also take the card out of the machine and use a modern computer to preserve its contents from time to time.
The linked SCSI adapter should let a mdoern computer use SCSI peripherals on a USB port, but won't let you connect your Quadra 650 directly to a modern computer, for file transfer.
One option that would do that is some sort of null modem serial cable. You could use whatever protocol you wanted (zmodem, ymodem, kermit, etc) to transmit files, but to be honest, that's a lot more fragile than most of the other methods, and will be a lot more difficult to restore or recover from, should something happen to go wrong. Plus, it will be significantly slower than LocalTalk, which itself will be way slower than EtherTalk using the 650's onboard Ethernet.
I would get a pi, put netatalk on it (maybe even just use a2server), and grab an AAUI transceiver. You can put these things on a switch and not connect them to the greater Internet, if you prefer.