Yep, something like Tasmota v8.4 from 4 years ago. Never bothered to update the firmware (Zigbee or Tasmota) on it since I was supposed to migrate everything to Z2M but didn’t quite finish the job.
Not really. The few devices I have on the ZHA network are mostly from Tuya (Aliexpress stuff) and ewelink (Sonoff, again from Aliexpress). The only device from a reputable brand is a leak sensor from Aqara.
As long as you follow the rules explained in this guide, especially the part which mentions Wifi interference, you shouldn’t be experiencing the issues you’re seeing.