What options do I have in terms of Zigbee controllers if using an old Readynas?

I want to use HA to control my Philips Hue bulbs. I already own a Philips Hue Hub but understand that using a non-proprietary Zigbee controller provides more flexibility.
I am not keen on trying to connect a Zigbee controller into my NAS server.
Are there any Zigbee bridge options that don’t rely on a USB dongle?

I have hacked a Lidl silvercrest hub.
There are several examples of other hubs that have been hacked too, like tuya and so on.
Check up before you buy and make sure that the version you buy is the correct one, since other versions than the one mentioned in the guides might behave differently.

Yes there are many standalone zigbee controllers that work with ZHA or Zigbee@MQTT. Have a look at the supported controllers from the Zigbee@MQTT docs.