Mah rating isn't everything. Pro 6 plus scores better in GSMarena battery test than Pocophone.
https://www.gsmarena.com/battery-test.php3
Also redmi note 5s camera destroys that if m6 note. Can't speak for redmi 6. Note 5 is known to have the best camera for a budget phone.
Its your own choice but I would never get the note 6. It's barely gotten any updates.
You might able to use the following app to circumvent the crashing settings menu? It'll allow you to access other settings menus or make shortcuts to them.
https://play.google.com/store/apps/details?id=com.sika524.android.quickshortcut
Using that App you may be able to get to the Apps menu by searching for "Apps Settings" under the Activities Tab and expanding the Settings menu. It's "com.android.settings.Settings$ManageApplicationsActivity"
Once you make it into the Apps menu I would probably go to the Settings App itself (System App) and clear cache / clear data / force stop.