Ball joint separation is rarely "easy"

, IMO.
Edit: I realize I wrote about the wrong joint (you are talking wishbone to plinth (which is what I think Lotus calls that thing that clamps the shims to the steering arm). The principle is the same but I use a bigger separator in this case a Kukko 129-0-25. You likely have to remove the plinth bolts to get the hub and upright out of the way.
I found those cast iron tools to be too crude and scary (I'm not surprised it broke) so I got a couple of German ones (you are in the right place). They are pricey but worth it. Pictured is a Stahlwille 12616 which is good for this small rod-end ball joint. Greasing the threads on the tool is recommended.
Lifting the suspension with a jack will open up the space a bit. Leave the nut on the bolt just far enough that the jaws of the separator can bear on the bolt the nut will prevent the whole arm shooting up at speed and smashing everything in its path. Once it is all neatly in place, tighten up (it will require quite a lot of load, enough to be nerve-wracking). Tapping (hitting) the side of the arm (not along the axis of the bolt or on the joint or tool at all) can provide a enough shock to break the bond. When I do the big joints in the cast iron upright, I hit it with a burst from the air hammer, and pop.
Good luck! I find it satisfying when they pop. 👍
PS: Is yours an Aluminium aftermarket steering arm (like the one pictured) or an OEM steel one? It will bear less abuse and may actually be tighter.