@bruceha2000
I finally had some time to do some digging, and according to the America's Test Kitchen GF book, potato starch "provides structure, tenderness, and binding power." According to some other books it gives breads a lighter airy texture.
Tapioca doesn't serve the same function but cornstarch, arrowroot, and sweet white rice flour can. According to ATK, arrowroot or sweet white rice flour can be subbed out for potato starch in "their" flour blend, which is the fairly generic rice/potato starch/tapioca mix.
I would imagine, if you are making up your own flour mix, the switch out with either mentioned above should work. I also grabbed a few books that have sandwich, brioche, quinoa sandwich, and a rye style breads that don't call for potato starch (or flour) to begin with if you want a copy. (I haven't made them yet and can't speak to how sliceable they are)
This site that lists all the different flours in a chart and covers the ratios needed so you can make up your own mixes according to what you are trying to achieve. I'm sure there are plenty more sites out there with the flour ratios.
https://theheritagecook.com/gluten-free-3/flour-power-building-a-flour-blend/
Came across this recipe in my searching. It's a millet based sandwich bread, without potato at all, and according to the blog looks to have a nice sliceable texture. (I haven't made it yet) From experience the best GF breads all seem to call for a little bit of vinegar in them, like this one, and I've always used ACV (apple cider vinegar.) I've done lemon juice too, but ACV seems to work better.
https://glutenfreebaking.com/gluten-free-sandwich-bread/
One of my books covers the ingredients and ratios too and I can copy those pages. And lastly, one of my books has all the different flours listed for each recipe and does not rely on one "base" mix. It's a newer to me book, but looks like it matches with other recipes I've tried. I only paid $4 or $5 online through a used online site.
Holler if you want copies of any recipes, the info from my ATK book on the potato/arrowroot sub, or more links for other books.