It might currently be a little on the hard side. I might up sales a bit to make it a little easier. It was recently a bit to easy in ver 6 but I might of gone to far the other direction, but we can turn all kinds of knobs until the right balance is found. That is a big part of why I have this open demo style of developing cause I need people like you to give me there feedback. I am also collecting your data via leader-boards and on my end I have it ver sorted so I can examine peoples play through to help me decide where balance needs to shift.
I totally agree about the lack of instruction moving to medium I totally get why you were like hmm its not working cause its not obvious to drag drop workers onto sliders unless you have played game dev tycoon which I based game-play around for small / med games. My plan to solve this is to offer a tutorial when you buy the second office and make it a choice to view. That way on subsequent plays you wont need to sit through it, but when needed I think a short guide explaining about drag dropping workers onto sliders and how they then take those workloads and how going over 100% workload on a worker will negatively impact your game, and also explain how endurance is the stat that directly affects how much workload you take from sliders. Training endurance isn't needed for medium although it makes it easier to set workers to sliders without hitting >100% , but it will be necessary for large / AAA.