Praise be to Allah.
There is nothing wrong with making software to estimate the costs of car repairs, even if the software will be used by insurance companies. Determining who is at fault in traffic accidents determines liability for loss of life and damage to property, and it is essential to figure out the cost of repair to damage or of replacing the car. This is something to be decided by people of experience.
So there is nothing wrong with making software for that purpose.
The basic principle is that liability is borne by the one who is at fault. If the insurance company undertakes to pay what is required, then there is nothing wrong with the injured party accepting that.
Based on that:
This software helps to ensure that the injured party receives his due, and that has nothing to do with the insurance contract between the at-fault party and the insurance company, or whether the insurance contract is Islamically permissible or unlawful (haraam).
If the car repair will result in the payment of taxes, there is nothing wrong with including in the software a feature that allows the mechanic to enter the rate of tax, because if tax must be paid, it will be the at-the fault party who will pay it, because it is part of the cost of repair, which he must bear no matter how great it is.
If your work is as technical support in car maintenance to estimate the damage to cars only, then this is a permissible kind of work, and has nothing to do with insurance or taxes. So you should not pay any attention to waswasah or doubts regarding it.
And Allah knows best.