Align buttons at the bottom of columns

you can position those buttons absolute.
Best would be to give to the color-section a unique ID – or a custom class.

The code below is with your default ID on this : #av_section_2 – but to avoid that every section_2 will do that on other pages – see above.

#av_section_2 .flex_column .avia-button-wrap {
  position: absolute;
  bottom: 0 !important;
  padding: 20px 0;
}
#av_section_2 .flex_column .av_textblock_section {
  padding-bottom: 40px;
}

The space now occupied by the button – must be added to the last element in front of it – here the text block – as padding-bottom.

Maybe you decide to give the headings a min-height for screens larger than the responsive case ( in your installation 768px )
f.e.:

@media only screen and (min-width:768px){
  #av_section_2 .flex_column .av_textblock_section h5 {
    min-height: 50px
  }
}

only if there are three lines of heading – this will not result in the alignement.