"For those who are arguing that it is impossible to teach primary maths to everyone at the same pace: that is how they do it in Finland, and Finland also has good maths results. From what I understand, Finnish teachers have also tended to stick to more traditional teaching methods, with little to no group work, textbooks/workbooks and whole class teaching."
Hmm--Finland's maths results as measured by TIMSS (the big international maths and science test) suggested that Finland's maths scores are about the same as those of the UK and US.
Finland did ace PISA for a few years, but the thing about PISA is that it involves almost no maths. PISA is fundamentally a reading comprehension test. The "maths" section in PISA is basically soft-skillzy stuff that involves spotting misleading graphs and saying why they are misleading, or working out which insurance scheme represents the best deal etc. There is almost much no algebra or trig in PISA.
That does not make PISA a bad test, it just means that PISA does not give us any insights into how to make students college ready for STEM careers.
But yes, Finland did tend to have quite traditional teaching styles up into the 90s or so, and still does tend to be more formal and traditional than Norway and Sweden.
Over the last 10 years there has been a gradual tendency towards Swedish-style "constructivist" ideas about education (group work, individual project based learning, students sitting around little learning pods doing stuff with iPads etc. etc.); predictably, the cohorts that have been exposed to more and more of this have done progressively worse at PISA.