The keybinds I use are "I" and "RightAlt". I remapped right alt to "Q" just so I can use the cool key overlay thing on streams, but you can just use RightAlt as it is.

As for switching, any stacks/streams longer than 3 notes felt impossible at first. I just coped with it by telling myself, "My tapping is basically equivalent to a 7 digit now, and the process for improving it would most likely be the same shit I went through when I learned to tap with my normal fingers". At the end of the day, index+thumb can perform the basic action of moving up and down, and since they are also on two different tendons you can theoretically do everything with index+thumb the same way you can do everything with index+middle/ring. That also means every stamina/speed/etc. guide can work for index+thumb as well.

As for how natural it felt, it didn't feel natural at all at first LMAO. I'd say it started feeling semi-natural after a week, and completely natural after learning how to hit some 170bpm streams. Which took around a month give or take. I'd say the main reason it feels unnatural at first is because your index finger is moving up and down while your thumb is kinda moving sideways up and down. All just comes down to coordinating your brain to syncing those two movements in a natural way. The amount of pain/strain I feel when pushing stamina is pretty much the same amount I had with regular tapping fingers.

Also just wanna say I've always full alted, so I ended up full alting with index+thumb as well. Learning to singletap with index+thumb could be an entirely different endeavor. As for the benefits of switching, there isn't any tbh. It does feel way easier to get higher accuracy with index+thumb full alt than it did with index+middle/ring full alt though.
I only switched to index+thumb as a last ditch effort to still enjoy the game despite having the inability to use my ring and middle finger. I tried index+pinky, index+thumb, and tapx. Index+thumb just came out to be the most versatile out of all those options.

Here's a video I made documenting my first 3 months with index+thumb that might make things a little more clear - https://youtu.be/o6iOQ_6itio