Westone UM2 or Shure SE530 (I think most prefer the Shure's

I have not heard them)
I am deliberately omitting UE as I see a number of threads about build quality recently. Of course you are more likely to see complaints than praise on the internet, but versus other manufacturers I err on the side of caution
For universal IEMs the UM2 and SE530 are at the top. After that you will need to enter custom territory from the £4-500 mark and upwards depending on manufacturer and model, and that is definitely entering diminishing returns
Let me put it this way. I am happy enough with my UM2 that I have no need to buy another IEM - I would only buy a custom, and that day will only be when I graduate, get a
career, pay off the student loans and can afford to flaunt it

If the UM2s break before all of that happens, I would either buy another UM2 or try the SE530 for 'curiosity sake'
Feel free to search for past reports about these two IEMs
Another aspect of course is the journey which you wish to skip. I bought 4 brands of canal phones (like your CX300), another IEM before getting to the UM2. Whilst it has cost me money and heartache, it has definitely helped me appreciate them more