Ok, what I think you need cannot be expressed purely in comments, so I have to write an answer...
Regarding the main question
First of all, both is important. As long as you have the feeling they (or some/most of them) lack foundations, you have to train foundations.
In training science, it is common knowledge that if you have mistakes in your foundational skills, you have to unlearn and relearn the whole technique. That's painful, it is boring, but it is important, because the patterns otherwise only go deeper into habit and the mistakes become even harder to get rid of. A good argument can therefore be "If you don't do it now, it only gets harder."
Normally, the idea is that you lay good foundations and then step by step add elements to it, may it be speed, resistance, combinations, partner's movement, etc. By that, you train, deepen and embed the foundations as well. Variability does not necessarily mean that you train different things - you may just as well think of different ways, e.g. with the help of using a movement as part of a game, or by using balls or weights. They must not even recognise it as training the exact same thing. Be inventive!
That means as far as possible, stick to repetition through new exercises!
But as this has limits, you will have to deal with the problem of people getting bored or annoyed by certain basic stuff.
How to deal with that
Two thoughts on that:
- Especially when only training once or twice a week due to time
constraints, an intermediate or advanced would not feel good when
doing just the same stuff as a beginner. My solution always has been
to show the beginners basic stuff, concentrate on only the most important points, let them start, and show some more elaborate stuff for the others. This does not have to be a totally
different technique, just e.g. more details and explanations irrelevant for learning a first crude idea of a technique, i.e. more sophisticated points to look at. Or higher speed. Or more
movement involved. Having to be attentive to other points basically makes it a new exercise.
- It is always highly problematic telling an intermediate or advanced that they do something badly or plainout wrong. Especially when they have a higher grade or can simply beat you up easily because of their strength/weight/attitude/whatever.
The latter point can be dealt with in different ways. One way for me has always been playing the dumbass. Like "Oh I thought this and that would be better, because of X?" Obviously works some times, but cannot be the only means. The best method turned out to be thinking of ways to validate my point. Like, when you have the feeling that a certain way would be better, let them train it, correct them, and have something at hand to test the success/difference. This may be a partner saying it had much more power, it may be a sandbag swinging much more, in the case of throws in Judo it was throwing someone the first time with ease instead of using strength, etc.
That way, it is not you telling them something they might simply reject, it is reality telling them you are right. And it deepens the trust in your abilities. And it enables them to see progress. I use this method for all my students, from four year-old white belts adoring you just because you care to over forty year-old black belts highly sceptical and probably grown up in basically a different art. And it works for all of them, because it is something defining their reality that cannot be ignored.
Main problem: Experience
First, to keep a group running, in 99.9% (the rest being high level competitors) it does not have to offer high quality training (though it helps), it has to be fun with cool people. It has to be a diverse training, it has to include some socials. Think of how you could train certain things you want to see conceiled as a game or partnerwork. Think of and organise socials. This it what keeps people coming. Uniform drills do not.
The second thing is you yourself. There is nothing wrong in feeling insecure in front of higher graded or stronger people or a bunch of kids. It is normal. And the confidence is something that comes with time teaching, with experiencing and coping with a great variety of problems of all sorts. If I would be asked to train a high level competition Judo team for one lesson in some days, I would shit my pants. I never did this. I know what to do, I know what to look for, but I would feel insecure as this isn't something I've done before. Give yourself time to become experienced. If you know many ways to train each technique and have seen almost every possible mistake and learned ways to deal with them (individually, not every solution works for everyone!), you become way more relaxed and develop an aura of confidence automatically. Sometimes I get told I have to lead the session the moment I arrive - and can make up interesting sessions for the group within minutes. That's not magic, that's experience.
You should really write down what you want to achieve, what is important for each technique (and level of experience ;) ), and a detailed plan for each session, if possible several weeks ahead. You will also have time to make up new methods of training. You will be able to think about what and how to say it. And you know how long you will train it (rule of thumb: about five, never more than ten minutes the same exact thing with the same partner - forms excluded). That way, you have something to test against reality and become more realistic.
Speaking of becoming more realistic, another part of getting experienced is how to set the bar. By this I mean that I always had very high expectations. Working with children and understanding limits of learning helped me lowering them. I guess one hard lesson every instructor has to learn is that most students do never achieve a high standard, especially not with 1-2 sessions a week. If the worst mistakes are out of the way, it is an achievement until middle grades. Tell them each time you see something getting better, only correct one mistake at a time (worst first!), and praise individually (I cannot stress this sentence enough!).
Additionally, it helps to widen your horizon if time allows. Attend seminars and trainings of other groups. Look into how they do things. It enables you to gain confidence about how it is right to be done over time.