TL;DR
Add an export to string and import to string button to each settings pane. These are separated per tab, so you can export startup settings, map settings or player settings. And import them too.Why?
Sometimes, when you download a new mod like Adaptive movement speed or To Do List, you want to test and change the settings on a new save file (so you can get a feel for the run speed, or the size of the GUI, or some other thing you need to be in game for (The Map (runtime-global) and Per Player settings).And then, when you want to start a new save file with those new settings, they are in the save, but the ones on the main menu/default settings are now "stale". So you have to manually copy over all of them.
It seems that even unchecking the "Use different mod settings per save" option doesn't make it so the changes we make in one game propagate to new ones, having to manually set them up one by one.
This is doable for one mod or two but it becomes cumbersome to do this for 10+ or 50+ mods.
A string export and import button for all of the settings makes this process automatic and helps with sharing settings with others (when you want to play "the same game" as someone else but aren't connecting to a server for instance).