In reality, they are scared of losing the position they have and feel vulnerable because they are not as critical to the company's core business product which is software I assume, and it is being written by you.

Talk to your manager and let them know you understand they are pressured too and that you are feeling pressure and try to find an alliance. Let them understand if they treat you well you will give them praise and credit for things that demonstrate to whoever is breathing down their neck that they don't need to worry.

Maybe they will stop. If they have no superior and this is their personality in general and it is a constant pressure style then you will be leaving eventually whether you think you should or not.

That personality exists in every occupation and field and nothing you can do, but try to take them less seriously and look for roles in your free time without telling them this. Make sure if you interview somewhere else you pay attention to the type of manager you may be getting at the new job. I've only encountered 2-3 really bad ones and now that I'm more experienced in life I believe there is a possible way to manage them in a way without making them feel threatened, and teach them that pressuring you will not work - either practically because it makes your work suffer or because you will stop being concerned by their pressure.

Hope this is helpful.