How do I send automatic Thank You emails for different donation amounts and donation types?
I've not tried this, but the Smarty mail template allows for conditional statements. So you maybe could create one email with different outputs, 

Some older tutorials:


https://wiki.civicrm.org/confluence/display/CRMDOC42/Smarty+in+mail+templates
https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=295109201


Another option could be to use CiviRules, which triggers actions at different points, e.g. send mail y when contribution of z amount is made:


https://docs.civicrm.org/civirules/en/latest/
https://civicrm.org/extensions/civirules