From your replies OP, I think there's a couple of things that stand out for me as a teacher, and they are things I'd point out to a parent if they ever contacted me concerned about their child's progres.
DS1 is sitting practice exams and getting 75% + so I know where / how he is dropping marks. Sometimes its lack of familiarity with the material / they haven't covered it / silly mistakes
75% isn't a child who is excelling at maths. Lack of familiarity would come with engaging with the content in class and prep so it is secure. Silly mistakes
If they've not covered the content, is that due to absence/cover lessons, it was available as extension materials but your DC hasn't completed it, was it deliberately unseen to push able mathematicians in the class to apply their mathematical thinking in a new way, or topics that your DC weren't expecting?
DS2 is a speed machine so will do anything to finish first but is regularly told to slow down and focus on quality. But I suspect he is turned off by the teacher (sorry this is where the boring problem comes in).Both kids also say the teacher is shouty (whilst boring!). Though the shouting in DS2's class has decreased since the start of the year.
You're putting too much on this when the real issue is the first part:
DS2 likely has quite a fixed mindset and views finishing first as a sign of intelligence. This is surprisingly common in students who have been used to performing well in younger years. They place value on the act of appearing first to finish, regardless of the quality of the work. It also means he is likely embedding misconceptions and silly mistakes because he isn't taking the time to understand and master the material.
Usually DS2 has lost interest by the end of the first sheet because its (in his eyes) too easy.
Again this is about self perception on their part and what they think looks smart. They've decided that because they think it's too easy, they don't need to bother with the more challenging work. A student who is making silly mistakes throughout their work isn't showing they've mastered the content. A student who doesn't do the more challenging work because they claim they're bored on the first part is shooting themselves in the foot.
The previous teacher had a style they both preferred where there were 3 different groups and each group started with a different level of difficulty
From what you've said I'm not surprised by this. One or both seem to make silly mistakes on work, rush through, want to finish first, don't do the challenging prep because they've decided the start is boring, so it's not a surprise that they'd find value in something that outwardly labels them as brighter.
There's an awful lot of focus from your DC on the teacher being boring, the homework being boring, but not very much from them on owning their own learning and their own attitude to their studies.
My advice as a parent/teacher to another parent would be to start with your children and say that you're happy to discuss their progress with the teacher, but only once their scores are consistently high, their prep is done to a high standard and they stop making excuses. I think they're trying to play you off against the teacher to be honest.