Been noting down some roadblocks I stumbled upon in the other replies. Unfortunately, swapping anything into RUNE on LeoDex on Spot didn't work for me, so I had to get some RUNE off a CEX. I could finally open a position and this worked nicely now.

There's one confusing UI glitch, though
When I look at the form to submit my order, the max amount typically shows my available collateral times the leverage (I used 4x) here:

Then, when I click on the max amount it would actually use the simple amount of collateral I have instead of 4x-ing it. Then in the confirmation popup it actually shows it would use 1/4 of the actual amount of collateral, whereas the order is then submitted correctly.