I always choose 12:35pm flight to SG and I took 9am direct bus from Genting to KLIA. So far no issue and have tried many times already. The bus will reach KLIA before 11am.
Instead of choosing direct bus from Genting to KLIA, you can take bus from Genting to KLS, then another bus from KLS to KLIA. Will be cheaper than the direct one and also the buses are like 30min intervals so more flexibility in the timing. Use app like easybook to book the buses. For the KLS to KLIA, I will book only when I reach KLS. Can still book just before the bus depart or buy the ticket at the counter. When alight at KLS from Genting, no need to go up. The bus to go to KLIA is at same level.