I literally only spent about 30 seconds in passthrough before i packed it away to be returned (dead pixels), i thought it wasn't as good as the various youtubers have claimed. I tried reading stuff on my phone, nope. But the lighting might not have been optimal, i've read it can make a difference.
Unless you plan on living with the headset on, and living through passthrough. I wouldn't worry much about it looking grainy etc. Once you start playing in MR, you focus on the game, not your surroundings. Soon as it's not the focus of your attention, your eyesight is pretty poor, and furthermore, your brain is doing most the work to fill in the gaps anyway. Passthrough is more than good enough for that.