My utility format the feed url by extracting the relevant suffix from the url slug when it includes:
"/user/"
"/channel/"
"/playlist?list="
I think the newer slug format for channels is "/c/" in which case I think you do have to extract it from the HTML source, but they now include the feed URL anyway.
https://www.youtube.com/user/mkbHD
https://www.youtube.com/c/mkbhd
For some reason though using channel_id=mkbhd doesn't work. This is a working link: https://www.youtube.com/feeds/videos.xml?channel_id=UCBJycsm...
It is embedded on the channel page as <link> element, your RSS reader should accept the web page link and figure it out.