"House rules" is the way to go. Over a dedicated ago, I abandoned the regular game for LLaMa. As you say, that can quickly get too easy due to poor AI squad building and the ability of LLaMas to purchase players too good for the level. So over the last few years I've combined LLaMa with Youth (Academy) - Only challenges. That is, you cannot buy or acquire ANY player who does not come through your youth academy. Given that at LLaMa level your facilities are minimal, and you have no money to upgrade, it is proper difficult!
It's best as a one-club career. Don't get attached to the players as if they're any good they'll get poached, but you can become a club legend over time.
EDIT: I just noticed who the poster is above me - his lower league file would be a great place to start! The Editors Hideaway is a great place for people like me who find the 6th tier not 'lower league' enough