The quick answer is if the site you are allowing is very simple ... yes it will work. If the site is like Facebook ... it is a lot more complex. the modern web is so distributed, allowing facebook[.]com will also require many more sites ... And even if you spend time to create that allow list, it may change. (CDN ... new software ...)
Even IoT devices difficult as well. We have one instance where a user blocked all traffic, and only allowed a few sites ... it worked for a while until things broke. The cause was NTP, the box can not get accurate time when NTP is blocked.
On Regional Blocks
Same as above, the internet is so distributed ... it may be dangerous to block larger countries. We have seen one customer blocked everything and only allow USA sites and many of the shopping sites were not working. This is because shopoify[.]com, which powers a lot of the online shops, is located in Canada.