Bulk upload of whitelist

This seems to be related to the inability to use custom lists due to performance issues. I'm guessing right now the limited possible combination of blocklists allow resolving to be done with minimal CPU & IO call, while users with the large custom lists would suffer.

Yes, that's a big downside to me. I whitelist a lot of domains manually and this is a major pain to have to add them one by one. That's a very basic feature present in almost every other ad blocking solution.

I absolutely love NextDNS but that's the only thing that keeps me from questioning whether or not I should try to switch a pi-hole or AdGuard Home. 

A feature that would also be great would be the ability to withelist a domain directly from the page we get when trying to access an address that is blocked. The administrator could enable this option to only specific devices/MAC addresses, or require to enter a password when trying to whitelist a domain that is blocked.

I think this feature would be a wonderful way to reduce the risk of new or casual users simply deciding to quit using the service  suddenly when they can't reach a website that they don't want to be blocked. 

I absolutely disagree, as long as you open whitelist requests to the maintainers of the lists you currently use in order to reduce and improve the contents of the lists

here here!! and by extension, Bulk Upload for DENY Sites would be great too!!!! (or need this be its own idea?)

Definitely a much needed item.