@all (@jolla4ever, @ric9k, @carmenfdezb, @ddelamarre), concurrently to trying to resolve this once more personally, I think a proper feature request should be written, which asks Jolla to unlock any encrypted partition on SD-Card when SailfishOS boots as they do with the /home volume (using the same password). As the topic is complex and most of us are not native English speakers, plus a proper feature request has to be well phrased and technically precise, I strongly suggest to draft that feature request here, using the information in this thread and from “Mount luks encrypted SD-Card automatically - #4 by olf”, “Automount encrypted sdcard with key file - together.jolla.com” (from 2019, “tracked by Jolla”), “Locking/unlocking encrypted SD card - together.jolla.com” (from 2018, the oldest I found, because SFOS 3.3.0 introduced the encryption of /home), “Unlock encrypted device and sd card together - together.jolla.com” (basically asking what we should ask for, again), plus naming these references.
Then we can perform a mutual quality assurance until we deem it to be ready to be posted in a new thread.
Whoever wants to start, please do first copy Jolla’s bug report / feature request form here and start to copy existing text fragments from these references to it (especially the last one above, it is a good starting point). Additionally put a list of these references at the end of the report.
P.S.: Actually I would like to use this thread only for that and then conclude it.