AirPrint with legacy Airport Express

I have found out that it will not work with the ethernet port

Works fine here. How old is your "old" AirPort Express? We ask because the Ethernet port is not enabled on older A1084 and A1088 versions of the AirPort Express. It is enabled on the A1264 and current A1392 version of the Express.

but how about the USB?

Sorry, but no.

It works fine when using a AirPort Extreme with ethernet

This is true if you have an AirPrint compatible printer.

but that option somehow makes my WiFi unstable.

Not sure what you mean by unstable. We need more details. The printer should work fine this way if you have disabled the WiFi on the printer so the only connection to the printer is over Ethernet.

(when using the Extreme as an "bridge" I does not allow the WiFi broadcast to be turned off)

Does the AirPort Extreme connects to your main router using a wired Ethernet cable connection? If yes, you can certainly turn off the WiFi on the AirPort Extreme.

If the AirPort Extreme connects using wireless, and you turn off the wireless, then the AirPort Extreme cannot connect at all.

Thought that was the purpose of the USB, but maybe not on this old one

AirPrint was not designed to work over USB. It was designed to work over wireless and/or Ethernet.

The Extreme is set "extend a wireless network", connected via WiFI.

When you extend using wireless, you will lose at least half of the potential speed on the entire network. And, this assume that you have the Extreme located where it can receive a very strong wireless signal. If the Extreme is located a few rooms away, it is actually doing more harm than good.

Always connect an extender using Ethernet if possible, since there is no signal speed loss at all through the Ethernet connection.

It seems I need to buy a brand new Express.

Or a used A1264 version. These are readily available for $20-25 U.S.

Not sure if you know that AirPrint is a generic std. For printing so ios which dont have device drivers can print on the printer

About AirPrint - Apple Support

Connecting printers to airports is not really related to the term airprint