SVGs are still often better subsetted raw ("unbundled") in that case both for local development and on HTTP/2+ servers. Things are slowly moving back away from bundlers/bundles which were mostly about getting the most out of HTTP/1.x tradeoffs most of which either no longer apply (because changes in browser caching rules for privacy reasons) or are mostly obsolete concerns (with HTTP/2+).
You aren't going to get easier subsetting in a modern browser than a folder full of SVGs and the browser only pulling the URLs it needs as it needs them rather than an entire bloated icon font format all at once up front. No need to double check that Calibre has got the right font subset from your CSS or to open Calibre at all, just upload the folder of SVGs and let the browser "subset" it, in the classic ways of HTML going back to its origins. Even less "automation" than Calibre.
You aren't going to get easier subsetting in a modern browser than a folder full of SVGs and the browser only pulling the URLs it needs as it needs them rather than an entire bloated icon font format all at once up front. No need to double check that Calibre has got the right font subset from your CSS or to open Calibre at all, just upload the folder of SVGs and let the browser "subset" it, in the classic ways of HTML going back to its origins. Even less "automation" than Calibre.