That shows it IS working. You have high phosphates and the pad is removing them.
Just an FYI: Poly filter pads can hold a lot so just because it turns a color on the outside does not mean it needs immediate replacing. What you do is make a slice around the middle of the pad and see how far into the pad the color has gone. If it's brown on the outside and white( ish) on the inside, it is still working. If the middle has also turned brown ( or another color depending on what it is removing), then it needs to be replaced. That's the miracle of the Poly filter and why I'm always recommending it.
As for the internal filter with just the polyfilter pad, that's a 50/50 idea. 1) You will have the cost of the pad ( I'd use the big pad and cut it to size to save money long term) vs the cost of dechlorinator and time doing daily water changes since the replacement water shows zero phosphates. 2) Then you have the convenience of seeing the pad in the HOB vs having to dig out the internal filter to see if the pad needs replacing or is still working. 3) Then you have the "I'll just replace the pad every X days/weeks" which means you could be throwing away working pad vs only replacing the pad when it is full. So your internal filter idea is good but I'd do that after the phosphate level is under control vs as a means to reduce it. But that's just me, I hate to waste $$$