Gog

If you're looking to level up before tackling the Temple, there are, I think, a few options.

First, if you killed Lareth, you can try selling his stuff to the Traders in Hommlet. I think this triggers a later encounter that's supposed to be a pretty tough fight. Possibly two encounters, although they may also end up combining.

Second, the fights in Imrydis Run are reasonably tough, although you say you did all the Nulb quests, so you may have done that.

Lastly, you can always just wander off to some place like Deklo Grove and keep resting. You'll find yourself fighting lower level monsters, but also may get some decent fights against trolls and such.

I basically did the above to bring my party from Level 4 to Level 6 after finishing the Moathouse, although I also did the New Content stuff (Arena, Welkwood).