Its worth chatting to the admin team at the uni about it if possible, @Blackcloud . If you can do a Leave of Absense without any penalty/downside (no student fee contributions, erc) then yeah I reckon it keeps your options open for when you finish your psych degree.
Finances are always an emotionally difficult thing for me too! However in this case, if it were me, I'd be trying to reframe it for myself as "I've already accrued/paid those fees. Nothing I can change about that." And then focus on whether there is any advantage of continuing/deferring vs formally withdrawing now.
E.g. if you dont withdraw formally, would it change whether or not you can transfer your completed subjects (eg as electives) into your psych degree?
If you do withdraw now but then decide you want to do an art qualification later, would it impact the ability to enrol at all? (If you have completed the psych degree by then, I doubt it would negatively impact your ability to enrol in another course - but might be worth asking student admin?)
I really hope you can get the subjects transferred over to be counted in your psych degree. Definitely not "wasted money" then since you would have to do units anyway.
Do you have many Electives in the degree?