Have you tried comparing a wet footprint with some of the images from various footbed manufacturers? It’d give you a start point for whether you need low/medium/high - leaving aside the differences in what is considered to be medium or high by different brands and how far forward/back they put the support.
I had a Retul fit a couple of years ago, and the last thing the fitter did was give me a pair of Specialized green (+++) insoles. Honestly it felt almost uncomfortable under my feet as the arches had never been supported that like before, but I felt an immediate and very noticeable difference in stability of the legs when peddling. Instead of my knees collapsing inwards the leg felt like it was staying well aligned for the whole pedal stroke.
My feet adapted pretty quickly to the sensation of pressure under the arch and now it feels strange NOT to have that, but worth bearing in mind that you may need adaptation time.