There are a number of suggestions for improving keyboard shortcuts in Proton Mail, but none that I have found that specifically address text formatting. In Gmail, I especially use the ones for Bold, Italic, Insert Link. Below is the complete list of formatting shortcuts from Gmail. It's a bit of a mare's nest and perhaps it would be best to provide the ability to customize shortcuts for formatting as well as navigation, labels, etc.

Gmail Formatting Shortcuts:

Ctrl + z : Undo
Ctrl + y : Redo
Ctrl + Shift + 5 : Previous font
Ctrl + Shift + 6 : Next font
Ctrl + Shift + - : Decrease text size
Ctrl + Shift + + : Increase text size
Ctrl + b : Bold
Ctrl + i : Italics
Ctrl + u : Underline
Ctrl + Shift + 7 : Numbered list
Ctrl + Shift + 8 : Bulleted list
Ctrl + Shift + 9 : Quote
Ctrl + [ : Indent less
Ctrl + ] : Indent more
Ctrl + Shift + l : Align left
Ctrl + Shift + e : Align centre
Ctrl + Shift + r : Align right
Ctrl + Shift + , : Set right to left
Ctrl + Shift + . : Set left to right
Alt + Shift + 5 : Strikethrough
Ctrl + \ : Remove formatting