Unfortunately, while Sieve is designed to be regular enough that tools can manipulate the language directly, the web UIs I saw when I last looked (a few years ago, admittedly) all maintained their own state, serialized into comments in the sieve scripts, and the actual directives would be regenerated from those, so that while Sieve remained a lingua franca for humans reading the scripts, machine-based manipulation interoperability was lost.
Note that while IMAP Sieve is good, having Sieve scripts run by the LDA from the MUA can let you reject without having to generate bounces locally, reducing backscatter. Eg, Exim's Sieve support with pysieved for a ManageSieve interface.
I'm comfortable working with config files directly, so just use sieve-connect to manipulate the scripts, but since there are desktop clients for Gnome and MacOS now, perhaps those are worth a look?
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