but I don't know what images at >=300% look like.
Well, there is an issue. Normally they should look good, at least up to a certain size...
But, currently, even just scaling the image to double size (which should look really great), does not work well: https://gitlab.com/freepascal.org/fpc/source/-/issues/33978
I found a solution for the FPCustomInterpolation up-scaling issue and added a patch to your bug report.