It looks like you need to read this. You need some background on what is onyomi and kunyomi and this is it.
Knowing which to use when is tricky because there is no reliable rule to follow. Well there are some rules, like if there are hiragana in the word or if it is a word with a single kanji it is often kunyomi. The lessons usually explain these rules. But there are so many exceptions. There is nothing to understand. Trying to figure out whether you should use on or kun adds a layer of complication. Most people end up just learning the reading that goes with the word without worrying about whether it is on or kun because this is what works best.
Most people don’t. Some do but they are outliers. Doing lessons generates reviews because every lesson you take becomes reviews afterwards. Doing all the lessons in one go makes a lot of reviews. It doesn’t matter much in the early levels but as you progress the workload will pile up. This is why most people spread out their lessons over time.
Keeping the apprentice count below 100 is a guideline about when to take lessons. Below 100 it is OK to take lessons. Above 100 it is best to hold off lessons until it is back under 100. The logic is that most reviews comes from apprentice items. If you have a lot of them you have a lot of reviews coming up. Keeping this count low is a trick to keep the review workload at a manageable level. It is not an indication of how good you do. So if you see your apprentice count around 100 it just means it is OK to take lessons.
Yes you did. You don’t feel like having learned something because you start to measure how much there is to learn. Japanese is a tough language for westerners. Learning it is a long journey because there is so much to learn. You had a good start but it is still only the beginning. You have to be persistent. Don’t feel depressed when you see the journey is long. Just consistently learn a bit everyday and things will become easier over time.