If you want to experience true frustration, try buying one of those 3d metal kits which require a set of pliers to assemble. The instructions are incredibly sparse and an incredible amount of precision is required.
If you want a bit of an increased challenge, Gundam models tend to be a bit more difficult than Lego while still feeling stimulating and fun.
I've built a few metal earth models. It is not very easy, but if you have a good plier set, perfectly doable! It puts you in a flow, just like with Lego, and the result looks nicer than Lego, for less money.
If you want a bit of an increased challenge, Gundam models tend to be a bit more difficult than Lego while still feeling stimulating and fun.