They say that "we do not (and will never) sell
As with any service I suppose it depends on how much one trusts them without having a mutually agreed to, signed and notarized contract that has severe penalties for breach. I do not know much about NextDNS so my question would be, how does anyone know for sure how long they retain query logs if at all and are any of their employees here on HN that could answer this?
If one could not answer this then perhaps a mitigating control may be to terminate DNS via DoH/DoT to a self hosted VM somewhere then use something like Unbound DNS to forward via DoT/DoH upstream to NextDNS so that they only have a random VM IP in their logs. One could also pre-cache the most common domains and many random domains hourly in a cron job to hide ones behavior. This method may allow for quickly switching to another provider in a centralized location without having to reconfigure all the individual devices.