I'm really enjoying the new Proton Sheets, great work!
The Conditional Formatting manager could use a couple of quality-of-life improvements to make it easier to work with multiple rules:
Currently, the Conditional Formatting sidebar lists every rule in the entire sheet, regardless of which cells are selected. This can make the list very long and hard to navigate in larger spreadsheets. Google Sheets only displays the rules that apply to the currently selected cells, which makes it much faster to find, edit, or reorder the relevant rules. Adopting the same behavior would be a big usability win.
Also, when editing an existing rule, it would be helpful to have a button or option to "Add new rule" that duplicates the current rule as a starting point. This is especially useful when you need several similar rules (e.g., different color scales or thresholds for similar conditions). Google Sheets offers this functionality, and it saves a lot of time compared to building each rule from scratch.