Is there any open source version of their RSS creator?

There are other opensource alternatives you might be interested in too [0], [1] and [2]. For rss-proxy you do not have to deal with selectors, though the feed quality won't be as good as the one OP suggests.

[0] https://github.com/damoeb/rss-proxy/

[1] https://github.com/RSS-Bridge/rss-bridge

[2] https://github.com/DIYgod/RSSHub

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