One thing I would do is plug a USB powered hub to the Extreme and plug the printer into the hub. The airports have never had much in the way of spare power for USB.. and crashing the Extreme sounds awful like a power issue.
Another thing to try if the hub does not work, is to setup the printer as IPP not bonjour. Printing directly to IP address is often more reliable.. You will need to make sure if the Extreme is setup as bridge device that it has a static IP. You can do this via the Extreme Internet tab directly.. although also doing it in the main router via dhcp reservations is also useful.
Sharing a USB printer is kinda trying to make a device that was explicitly designed to ONLY work as a local printer into a network printer. There are many slips when you attempt to do this.. and the reality is networking (usually wifi) in printers is now so common if you want a network printer you should really buy one. Printers are cheap.. a holder for expensive toner or inks that companies can sell you over and over. Instead of buying a new toner for your printer next time.. buy a new printer.
USB printers have great variation in the way they perform as a pseudo network printer.. basically the print engine only with the page makeup done purely in driver.. which was never designed to work over a network. The USB printer manufacturers have worked to make this impossible.. and the router designers are trying to overcome a deliberate limitation. Each is fighting the other.
Sometimes it is worth going back and seeing how well it worked for people in the past.. here is a post from 2002 about sharing the HL-1440 from an Emac .. now that goes back a ways.
https://arstechnica.com/civis/viewtopic.php?f=11&t=750385
If you have multiple computers you can share the printer from the Mac Mini.. that will work a lot better as a print server than extreme. Otherwise it is a very old printer.. and even if it works great still the consumer world has decided it is time for you to buy a new one.
