Tough crusts can be an issue with sourdoughs. Maybe try something slightly fresher like your poolish or a biga plus the addition of a cup of dough reserved from a loaf you baked about a week ago. Have you done any old dough yet?
Oven temp can be a really big thing. I would get an accurate oven thermometer and find out what your oven's true temperature is. +-25 degrees is kind of the norm and it is not unheard of for ovens to be 50 degrees out.
Do you measure by weight or by volume. Flour is highly hygroscopic, so a change in humidity can give you wildy different results if you are measuring by volume. If you are close to the ocean or in a very humid area most recipes aren't designed for you. I have the opposite problem. I live in an extremely dry area and in winter measuring by weight can yield me a full cup less of flour than it does in summer.
Do you use any of the steam oven techniques? Ice cubes, spraying the oven, baking in a Dutch oven? My preferred method is a baking stone, I use a big heavy Pflatzgraff that we bought in the mid 90s, and a giant stainless steel bowl. The circumference of the bowl needs to be smaller than the circumference of the stone but big enough to not interfere with the oven spring of the dough. On the other hand the benefit of a Dutch oven is that you can make a really wet dough and if your Dutch oven is properly sized it can't spread out too much.
Have you ever tried any fridge aged, no knead recipes? They tend to yield really nice, thin and crisp crusts. They are not sourdoughs, as such, but because you can age them in your fridge for a week or more they do have a complex and slightly tangy flavour. These can be quite lovely loaves. I go through phases where I might bake no other breads for months on end.
Do you use any kind of dough conditioners? Sugar, milk, egg or any of those baker's conditioners will make your crumb more tender but will also hinder a thin , crisp crust. Having seen a couple of your loaves I am quite sure this isn't the problem, but feel I have to ask.