I passed ST2 on my first sitting, but have struggled with ST1/SP1 for a while now. I'm sitting SP5 tomorrow - it's much more relevant to my work (which helps greatly) - I do day to day some of the stuff that is covered there (perf attribution, yield calculations, risk adjusted returns, rebalancing strategies etc). Hopefully that will be of some assistance.
There is a higher mathematical content than most other SPs (barring SP6). Make of that what you will. If you liked CT8/CM2 then that might nudge you in SP5's direction.
The perceived wisdom of doing SP1 as it's so similar to SP2 didn't work in my case.
But, as others have said, it depends on the individual and their personal circumstances. Speaking for myself, I'm kicking myself for not having switched sooner. Two years ago, I switched internally in work to the investments area and I should have switched exams then, but I fell afoul of the "sunk costs" fallacy which saw me grimly attempting and re-attempting an exam I only ever had theoretical knowledge on but had spent a fair bit of time studying and I didn't want to see it go to waste.