Changes in SailfishOS 4 broke crypto-sdcard in (at least) three ways. Two breakages have been fixed, for the third one I have an idea what could be the cause.
But In became tired of this, because there seem to be only about 5 users of crypto-sdcard and such breakages are regularly happening lately with new SailfishOS releases (that was not the case with SailfishOS 2.x and early 3.x releases).
As long as I see no incentive to upgrade my “production-phone” to SailfishOS 4.x (there are too many things broken for me), I rather care about things which are more fun (e.g., Patchmanager).
But I can assure you, that crypto-sdcard is (still) designed correctly (i.e., there is no easier, proper way of achieving this, even though it may seem complicated on first sight) and I am sure willing to review and integrate contributions.
BTW, years ago Jolla stated they have auto-mounting encrypted (partitions on) SD-card on their ToDo list and to me the manual mounting was only an intermediate step to achieve this (IMO manual mounting is useless), but … it is Jolla.
They also have the ability to use their mechanisms to unlock and mount the encrypted home partition using a user-supplied passphrase (which is a proper solution); but this code is proprietary, so crypto-sdcard cannot use these mechanisms. Only with a self-compiled SailfishOS (i.e., a “community port”) one can change these things for the better by completely re-designing and re-writing this functionality, as rinigus did.
So either wait for Jolla to provide a solution or contribute a fix for crypto-sdcard on SailfishOS 4.