Do you mean keys thst you hold down with your thumb while your fingers are on the home row or something else? For the former, I use split space keyboards and map the left one to enter on press, layer on hold, the right one to space on press, and a different layer on hold. That gives me more keys that you would get on a 70+ keyboard. I find this considerably easier than trying to use pinkies on shift and symbols on what is my top row, qwerty row.

Typically, I map tab and the traditional enter key to tab on press, another layer, and backspace with the same layer as tab. This layer I use for window management in sway for home row and anything else I am missing that I really want.