First note that if you have problems specific to Zigbee2MQTT then it will usually be a much better idea to post to Zigbee2MQTT’s own community → https://github.com/Koenkk/zigbee2mqtt/discussions/
This Zigbee sub-forum in Home Assistant’s community forum is more about the ZHA integration which is built-into Home Assistant core as well as general discussions about various Zigbee hardware devices.
You should just need to wait, Zigbee devices should automatically find the best path, so just give it time.
It will usually take at least 24-hours before the whole network begin to settle down and can take longer.
Each device normally reassesses potential connections and switch to best route once every 24-hours.
General advice is to just add more Zigbee Router devices and do your best to avoid EMF interference.
Suggest that you start by taking active actions to avoid EMF-interference. Zigbee devices are very sensitive EMF interference and have a relatively short range, thus it is highly recommended that everyone using Zigbee devices try to follow all the tips here (especially be sure to connect the Zigbee Coordinator adapter via a long USB extension cable to a USB 2.0 port and not a USB 3.x or USB 4.x port) → Guide for Zigbee interference avoidance and network range/coverage optimization
Also while not a must if you have enough other Zigbee Router devices it might be worth considering adding a few known good dedicated Zigbee Router devices (e.i. dedicated Zigbee signal repeater/extender products) to act as the stable backbone of your Zigbee network mesh. When everything works well then you just need to add more Zigbee Router devices to extend range and coverage.
For example, the “IKEA Tradfri Signal Repeater ” and “Aeotec Range Extender Zi ” are products that work very well out-of-the-box, while a more powerful alternative is to make yourself some semi-DIY variant by flashing the correct Zigbee Router firmware to Zigbee Coordinator USB dongles and then using them as stand-alone Zigbee Signal Repeater devices in USB-chargers for power, like, for example, either the Sonoff ZBDongle-P (CC2652P based) or the Sonoff ZBDongle-E (EFR32MG21 based) work great as dedicated stand-alone Zigbee Signal Repeaters with recommended firmware.
I also suggest that you consider upgrading the Zigbee Coordinator firmware if you have not already (note that there are different variants of Sonoff Zigbee dongle; the “ZBDongle-P” and the “ZBDongle-E”), and also take note that currently only the “ZBDongle-P” variant is recommended by the lead Zigbee2MQTT developer because “ZBDongle-E”) is still missing backup and some other small stuff to have feature-parity.