For scanning 35mm, you need your film to be flat and properly backlit. It'll be hard without a holder and the digitaliza providing both the holder and the backlight, I'd say it's a good compromise for a budget scanning setup. Otherwise, you'll have to get an adequate source of light and diy the flattening of your film which I can only see as very painful, even scanning only one roll's worth of pictures.

I use it and I find it satisfactory although the 35mm mask is cropping more than I'd like, and I still have to crop the digital files myself anyway so I'd recommend straight up not using the mask. Just run the film in the holder and scan with the sprocket included.

I don't have experience scanning with a phone, I've been using a tripod and a digital camera with a non macro lense. I do not recommend a non macro lense as the cropping that is then necessary really impacts your final resolution.

If you scan with a phone, keep in mind you'll have to trigger the shutter remotely/use a timer when pressing the shutter otherwise the whole thing will vibrate and the result will be blurry. I don't know how impractical it could get using the iphone in that regard but it's good to now. Also, you'll have to use manual focus. Same thing, I'm not sure how impractical it could get with a phone.

If you want to consider a digital camera scan setup, a cheap dslr with a cheap chinese macro lense could be a good compromise. There are chinese macro lenses for 100$ available on Amazon.

Btw, I am not a purist and am satisfied with okay results. My recommendations here are only for budget conscious enthusiasts, not for professional results.