It's not as bad as you'd think. I got it on my first attempt after playing through the game normally a bunch of times. Knowing where things are and what fights you can take in which order keeps things manageable.

And dream fights also do not kill you. So some of the toughest fights in the game will not end a steel soul run.

Plus 100% in Hollow Knight isn't everything. There's a total of something like 113%, so you don't need to do some challenges you know you might die to.

If you can do Steel Soul, the 100% is not that hard to get. You get the Fragile Strength early on which is a pretty big boost, and since you never die, you never have to worry about losing it. Most of the bosses aren't a big deal after you've beaten them a couple times, the only one I was worried about was Grimm, and since you can get up to 112% you can skip some of the more dangerous threats, mostly from the DLCs.