You can use the hubitat and integrate it to Home Assistant using this custom component
Otherwise, since you are in the EU, there is no combined zwave/zigbee stick that will work there with your frequency. If you don’t go the hubitat route, you need separate zwave and zigbee USB sticks.
You also can re-purpose those PI’s and run the zwave and/or zigbee networks separately from Home Assistant on a different PI using https://zwave-js.github.io/zwavejs2mqtt/#/ and/or zigbee2mqtt https://www.zigbee2mqtt.io/ which both can be run on their own in Docker. They then can connect to your Home Assistant instance on another machine that is anywhere on the same network. So, you basically make your own “hub” that way.
As far as hardware, any of the “recommended” adapters on the zigbee2mqtt page should be fine - Supported Adapters | Zigbee2MQTT . I personally use the electrolama zzh stick, which is based on the CC2652 chip. Don’t get the CC2531 chip as it isn’t powerful enough.
I have an Aeotec Zstick Gen5 which has worked great for me, but be aware there are some compatibility issues with certain Aeotec sticks and certain RPI’s. https://forums.raspberrypi.com/viewtopic.php?t=245031