Hey there! I'm currently in my 1st year of A Level Maths, and so far, it hasn't been toooooo difficult in my opinion. The only thing I found relatively difficult is the trigonometric identities, and some of vectors.
And to answer your question about practice, yes, practice, practice, and guess what? Practice. Maths is all about understanding the questions that are given to you, but in A Level they get more complicated so you have to practice a lot to tackle each and every question.
And to answer you second question, yes, there is a lot of modelling. In fact, they are adding more modelling because the exam boards felt it's better in order to get students more ready for the real world. Don't let that bother you however, because modelling questions are always difficult no matter what, but you have to just think of the things you think about in pure maths, and try your best to know which technique to apply. There's 3 papers in A Level Maths, like GCSE, but paper 1 is just pure maths, paper 2 is statistics, which is the graphs and diagrams and stuff like that, and paper 3 is mechanics, which is the physics aspect of the course, and is usually considered the harder part of A Level Maths because the questions can be very wordy.
Now it may sound scary, but practice makes perfect, and Maths is a great 3rd option for you with chemistry and biology, as there's quite a bit of maths in that too, and maths will help back that knowledge. I take economics and computer science alongside it, and I personally think so far it hasn't done much to help, but it will in the 2nd year and the future as well. Especially for you if you're going into biology or chemistry for University or apprenticeship. Maths is essential for those. Good luck with your options, and I hope you do pick maths, because it's actually a lot of fun, and it's interesting, and remember to practice
