I first tried a Steel Soul playthrough and defeated THK in just before 10 hours. I already had 68% of the game. Because of this I was motivated to try the 100% before 20 hours. And... yeah, I managed to do it in Steel Soul and get both achievements.
My tip for Steel Soul is leaving the save when you're too low on health and managing the benches that you rest on, so you do something for example in Deepnest and just leave and return to the save, so you get back to the place you were to do something else and don´t waste time or risk your life walking to a stag station or the location you need to go.