1. Change the value of config_error_display to "0" in ocart.sql
2. Have disabled upgrade in 2.1.0.1 due to Errors in 2.0.3.1 to 2.1.0.1 upgarde and few tables are not generating after upgrade. ( same issue for 2.1.0.2, In next release plz check upgrade from 2.0.3.1 and check DB changes. ). Have mailed vendor.
3. Try to add upgrade from 2.2.0.0 to 2.3.X.X. We have disabled it in 2.3.0.2 because front link is not loading due to change in extention path. http://forum.opencart.com/viewtopic.php?f=183&t=165958
4. No Documentation available for 2.x to 3.0 upgrade.
5. PHP REQUIREMENT LINK: http://docs.opencart.com/requirements/ and also check system/startup.php(php 8.1+ required)
6. Move system/storage directory outside of web accesssible since 3.0.1.1. This was asked to do in manual installation post first login. Remove this folder from ocart2.zip and make data.zip. Also make changes in both config files as well for "define('DIR_STORAGE', '[[softdatadir]]/storage/');".
7. We have made directory system/storage/logs,cache,session in upgrade because the installer was asking for the same while upgrading the script. Opencart detects this storage path only while accessing installer, rest of script will access [[datadir]]/storage path as defined in config file.
8. If you add any new language then update its entry in install.php array.
9. Download language from https://www.opencart.com/index.php?route=marketplace/extension&filter_category_id=2 for opencart 3.x.

Consider following while making language packs.

	1. Download Russian pack from above link with name "Russian language pack v 2x and 3x and 4x"
	2. Download Turkish pack from above link with name "OpenCart Türkçe Dil Dosyaları" (Extract the package and make the zip according to our format )
	3. Check the structure of the admin and catelog packs with the one we provide.
	4. Check all the language for every new version.
	
10. There was an error on installing 3.0.3.7 version on mysql 8.0 and also on upgrading opencart to 3.0.3.7 hence we have made manual change in /system/library/db/mysqli.php as suggested by vendor.
https://github.com/opencart/opencart/issues/9075#issuecomment-774502931