I forgot about my promise to post about release of Smart RSS v2 on Vivaldi, so sorry about the delay.
As always you can download my extension from addons catalog right here:
https://addons.opera.com/en/extensions/details/smart-rss/?display=en
Dev. builds of v3 can be found on my official blog in comments:
http://blog.martinkadlec.eu/post/501-smart-rss-final-v1
Nice! 😀