You asked: “Do I turn strict on them?”
Yes, absolutely. It’s obvious they’ve been taking advantage of your good nature.
They are old enough to know better. Require more of them.
You said you’re afreid it will be too hard for them. Huh? How is requiring them to act like polite adults being too hard? Stop being afraid of whether they will like you or not. You aren’t there to make friends. You are there to teach them; not just course work but how to behave properly in the adult world in which they will soon be expected to earn a living.
Treat them as adults and require that they give you their best effort both in conduct and in course assignments.
Regardless of age level, it’s a basic truism that students will get away with as much as you allow them to. Stop allowing it.
Obviously it would have been better if you’d have started off strict to begin with (and next year you should) but it’s not too late to set a standard and hold them to it.
Just warn them in advance and let them know ahead of time that things will be changing. Since they abused their freedom, this is the result.They will know exactly what your talking about.
And then stick to your guns about it. Don’t be apologetic about it. Set the rules firmly and don’t vary from one day to the next. Consistency is key.
Once they know they can no longer get away with this crap, you’ll be surprised at how quickly they shape up. But you must be consistent about it. Otherwise it’s just chaos.
Even tho the age group I taught was younger, my basic attitude was that I cared little whether or not they liked me, but they would respect me.
But respect is not automatic. It is earned by being fair, impartial and consistent about standards and discipline. The sensible ones will like you for requiring more of them and the class as a whole. The rest don’t matter.