Minimizing reflection coefficient of a low-pass filter
If you are building a lossless filter, the only way that you can prevent power from going to the load is to reflect it back to the source.  And a filter's response is continuous with regard to frequency (and keeping in mind that a dip down to \$-\infty\$dB is accompanied by a phase reversal, so the actual number is going through zero in a continuous manner).

So yes, you're fundamentally approaching this in the wrong way.  If you want to maintain the load impedance on the source within limits then you need a diplexer, so the out-of-band energy from the source gets burnt up in a load resistor.  This is not usually a thing for transmitting applications -- the only place that I've seen it (in my admittedly hobby approach to RF design) is between a mixer and an IF strip in a radio designed for the lowest possible 3IM products in the mixer.