The Maybank Premiere cards typically have 30-40% off but the limit is up to 10 if I recall. Works at all Starwood and Shangri La KL hotels. I've also tried Grand Hyatt and Mandarin Oriental buffets, but unfortunately most 5-star hotel buffets are far more than RM100 now. MO breakfast buffet alone was RM140 at one point.

You didn't like Lemon Garden? I thought it was pretty good. One option is DoubleTree KL's buffet, you can get discounts to get it below RM100 per pax. It is at the same standard as Lemon Garden though.

Personally I very much enjoy Hotel Bangi Putrajaya's Kazumi japanese buffet dinner or their dim sum lunch at Harmoni restaurant. Both are RM95 net if I recall, but they are only on Saturday dinner and Sunday lunch respectively, not during weekdays.