It might vary in play now. I only saw a couple of fumbles in my entire week 1 game. Give it an honest shot. I feel as though the sliders in this game don't do as much as global game speed and speed parity. The sliders seem to be more of an assist to global game speed and speed parity. On All pro, with everything at default (sliders at 50/50), it makes sense that true player/ team ratings matter. I do like having the QB accuracy slightly lower for both.
If you want a challenge, play with a garbage team. Or use your team or a team you like, but get rid of your superstars. Or if you really want to use a good team with the players on the team, you can lower specific player ratings on your team appropriately. There are options for you.
All pro, default (50/50), QB accuracy 47/47, fast global game speed and speed parity of 90 or higher (I like 90) is the way to go. No assists, except for defensive switch assist, and make your own house rules. That's the way to go! I always play with a defensive end on D, but that's because I always have. I do use Coach suggestions for play calling, though I have no problem diving deep into my playbook. It's good to mix it up. Use different formations and sets. Also, don't be afraid to change your scheme and/ or the playbook that you use before each game. That will keep it fresh. For instance, in my first game, I used a west coast offense for scheme and an attacking 4-3 for defense. I used Chip Kelly's playbook (SF) for offense and Jim Schwartz's playbook (PHI, Doug Pederson) for defense. I'll probably keep my defensive scheme the same for the entire season since those rarely change mid season, though I may vary the playbook. The offenseive side of the game is going to be a free for all. I can see a game where I switch to a spread and use New England's playbook, and a game where I use a power run with Kyle Shanahan's (ATL) playbook. Mix and match as well.
If you just do coaching suggestions every single time, you'll get the same handful of plays suggested to you. There's nothing wrong with that however.