cbwjm
Seb-wejem
I'd say run with the one spell per long rest. I don't think that would be really all that unbalanced. If you are worried about them completely changing their spell list around when they have a couple of weeks of downtime, then you could grant these class a bonus spell known (or even just designate one of their starting spells known) which is the only spell known that can be changed during a long rest. This means that most of their spell list is set (apart from the level up change) while still giving them a little bit of flexibility.