Am I allowed to use software tools that I created on company devices and outside of working hours, and benefit from their ideas and the customer data they contain for my own private business, knowing that my use of them will not deprive my previous work of them and will not harm the company's interests?

The questioner is thanked for his keenness on what is permissible. There is no harm for him in benefiting from the private software tools he created for his previous work, as their intellectual property belongs to him, as do the ideas and programming techniques he learned through his previous work. As for benefiting from what belongs to the company, such as its secrets, plans, database, and customer lists, none of that is permissible to benefit from without its permission.