Start by reading and try following all the best practice tips in this guide before troubleshooting any deeper (including using a long USB extension cable tpo a USB 2.0 port as well add having/adding many good Zigbee Router devices):
ConBee can not generally be recommended for Zigbee2MQTT or ZHA (instead only if use DeCONZ or if happen to own it before migrating), see → Zigbee buyer's guide - #2 by Hedda
Of couse also see Zigbee2MQTT documentation which indirectly say same thing (with various important features not being supported by ConBee but are supported if using ZStack or EmberZNet):