Alternatively, you can use a taller TROFAST unit as the final step, alongside the staggered unit. The 5-bin TROFAST below measures 145cm in height. Cut the two sides down a mite if you want to lower the step.
TROFAST unit | IKEA.com
You’ll lose a bit of storage but you can still insert a shelf or shallower bin in. Do secure it to the TROFAST step unit. See this tutorial on a TROFAST sturdy stair conversion for details.
Option 3: DIY loft bed stairs
Lastly, you can build a ladder from scratch. You may be able to salvage the STUVA steps and attach them to the side posts of a new angled ladder.
Cost wise, this is probably the best way to go. Get the side planks cut at your local hardware shop, if you do not have the tools for it.