It all depends on what long term solution you want to implement and how much you want to spend.

All the options you suggest would work but the most elegant solution I think is to put in a Apple LocalTalk to Ethernet converter/bridge onto one of the older Macs and hook the rest up via LocalTalk cabling. So now you have a bridge between your LocalTalk network and ethernet network. The downside is – those type converters are neither cheap nor easy to find.

The SCSI zips, removable media/external SCSI HD/floppy drive is really just a fudge to get round the problem - it's not ideal and will be inconvenient to use, but if you only need to do it now and again it would work.

I use a USB flash card to move files from my OSX machine to my older PowerMac 9600 which has a 2 port USB card in it, from there I can either put them onto floppy or hook the 9600 into an ethernet network and move files that way.

Anther alternative (that may well prove cheaper and easier to find than the ethernet to LocalTalk converter) is to simply buy an older Macintosh which has both LocalTalk and either an ethernet NIC or onboard ethernet which could effectively act as your LocalTalk to ethernet bridge. You simply hook the machine into the ethernet network and the LocalTalk networks - you may have to switch AppleTalk via LocalTalk to Ethernet and vice versa but it's still a far easier prospect than having to keep plugging and unplugging external drives.

I think there's also a software implementation of AppleTalk over LocalTalk to Ethernet conversion which could run on that machine, but I don't know much about that to be honest.

You've got a number of options, which you choose will depend ultimately on what you want to do.