PS To save you having to mess around further, you may want to choose the key slot encryption parameters at the time of first encryption. That is, if you use the example command as shown then you are taking default parameters for both the disk encryption and the key slot encryption. The default for disk encryption will likely be appropriate (based on recent testing, and only in my opinion of course) but the default for key slot encryption may not be ideal for the Librem 5 (although “ideal” is in the eye of the beholder). Refer e.g. Our LUKS fde may not be enough anymore - #66 by Loki and discussion around that post.
However it’s not that big a deal. While (re-)encrypting the entire disk is going to be slow, changing the encryption of the key slot after the fact is quite quick.