You'll have to kill the grass first in order to not have it take over again, the flowers soon die out. With environmental conditions present here tending greatly to favor the prevailing of grasses over flowers - all the more so because of all the weedy Eurasian grasses that are around. So that even in the minority of local sites such as bedrock outcroppings and certain other naturally treeless locations where shallow or dry soils have allowed the persistence of numbers of native meadow flowers invasion by weedy grasses and other threats is often occurring.

As it happens I did a garden consultation this year in Skagit County, Washington where the client had had the same idea as you. The wildflowers had been planted and were flowering in the part of the field where the septic was. Vigorous grasses were present all around the planted section, I don't remember what the preparation method was but it certainly looked like the scheme was not going to last unless periodically redone. Also the "northwest wildflowers" were all kinds like might be seen growing natively in the Siskiyou-Klamath area or even farther south in California; at least they weren't the various Old World species typical of wildflower mixes offered and planted in our region. But the point remains that checking to see which particular species a mix being considered contains is an important aspect. At least where a body is trying to support the local ecology, as you apparently are.