To deal with layers, I make it a policy that whatever button takes me into a layer is also programmed within that layer to take me back out again.
My opinion is that the 4 small buttons on the thumb clusters are useless. They are impossibly awkward to hit with your thumb. They could either be used for seldom needed thing or else just left unmapped and unused. Or perhaps all the buttons in a cluster could be programmed in the same way to allow clumsy jabbing.
I may at some point abandon the number row. Once a numeric keypad is set up in some layer, my hand could stay on the home position and I could learn to use the numeric keypad. The symbols generated by shifting could be mapped to the other half of the keypad to be used in the same way without long reaches. The number row could be left alone and redundant.
The idea of having a "mouse" layer on the keyboard is completely lost on me. I have a perfectly good mouse that is far more ergonomic that paddling around using the keyboard. Maybe I am missing something, but I don't think so.
Do a search "ergodox layouts for coding". Maybe also search Iris and Keychron.