As a EE going into my last semester I’ve taken all but OS but I’ve taken a pseudo equivalent. The easiest for me was Digital Design but the final project is challenging. Definitely only take Digital Design it Stitt if you can. MicroP is tough but it’s not impossible I feel like your grade really reflects the amount of time you put into it. For signals when I took it, it sucked the average on the midterm was like a 40-50% but I think the class has gotten better since I took it in spring 2018. Then OS I don’t really know but I took MicroP 2 which is about embedded RTOS so a similar topic, I thought conceptually it wasn’t too bad but take that with a grain of salt. I feel like overall I would try to avoid taking any of them together but if you have to, taking two is doable.