I've delayed a couple myself (I've two Kingdom Death minis sitting in the sock drawer). That said, I've found it is good to push into miniatures that have a higher level of detail.
When I started some months ago, I did so with some 30 year old Grenadier miniatures. I love these things to death; however, it is also true that they tend to have less detail than their modern cousins. Painting a miniature with a heightened sense of detail can give you more "targets" to paint and can help to challenge you to new levels.
That lich is definitely a challenge; but maybe you can find something in the mid-range, that's a bit more advanced than those orcs, but not to the level of the mega-lich? You've done a great job on those initial guys that you should definitely push into something a bit more detailed. Some of the Reaper minis are actually pretty nice and have more detail than your orcs - perhaps give them a shot! Also, I've been enjoying Otherword Miniatures, which appear to be a bit of an unsung hero as I don't see them mentioned very often. Otherworld seems particularly good at old school AD&D cheese, but well detailed. Their otyugh is awesome!
