Wax based paints need NOT be complicated, as long as you aren't wedded to "encaustic" techniques. In other words, Ceracolors, which are water-soluble wax based true paints with high chroma, are incredibly easy to work with en plein air. No heat source required at all, unless you want to set the paint layers. They behave very much like open acrylics, in that you can open the prior layers with water. You can use any style or technique that you would normally employ with watercolors or acrylics. I'll attach a couple of examples of my en plein air work in Ceracolors. BTW, people will be very surprised, as these aren't widely known.
Otherwise, I find it easy to work plein air with any combo of: acrylics (I like Chroma Atelier Interactive best and can imitate oil rather successfully), watercolor, gouache (I like Turner Acryl-Gouache, an acrylic based), pastel pencil, watercolor pencils (Inktense and regular), ink in line and wash (I use Faber Castel brush pens), and Ceracolors.
Here's an example of a ditty done with multiple media: ink, Inktense, gouache, watercolor, pastel!