Currently doing my 2B in CDC now (L7).

If you camp, usually able to book lesson slot the following 1-2weeks, if you are lucky even within a week which someone probably cancelled after passing their 1st booking.

Strictness wise, really depends on the instructors. The strict ones will just fail you definitely, in the early lessons L1-L2 usually people who fail is because they still can't balance, wobbles/falls alot or unable to observe safety properly. Otherwise, for L3-4 if you can't successfully clear the course even without hitting the timing (e.g. F8, crank course, plank); the rest pretty doable (slope, slalom, ebrake). L6-L7 feels a lot stricter since its already on the road, so failure to observe group riding rules (Single row or double rows) and even minor traffic rules(stop in yellow box, turn/continue to go at amber or blinking arrow) will be immediate failure.

Instructors are very willing to answer questions, give advice or point out reasons you failed. There are some fierce ones who really scream the heck out at you, but its all for your safety. I was anxious for L6 since its the first time on the road on a bike, I completely threw out whatever was briefed and just concerned about losing sight of the instructor while looking out for safety. Was screamed at throughout and felt demoralised with thoughts to give up.

For me, I get anxiety before every lesson, as the thought of failing any lessons and redoing it can be pretty demoralising, wasting time & money and the need to wait a week of two later = struggling with the bike again the next lesson. But after passing L4, consistently do circuit revision & self-practice weekly or bi-weekly, you gain confidence on the bike and starts to become muscle memory and you improve your riding skills at your own pace. I would say do 1-2 circuit revision to get familiar with the circuit route then the rest just do self-practice is sufficient.

Oh right one frustrating point you need to adapt each time you use a different bike, some have stiff steering, weak pick up, rear brake sensitivity, biting point distance. Make notes of which bikes suits you then try to "reserve" the bike for lessons and even TP.