Will using iPad Pro as monitor for my mini pc cause damage?

There are 24" PC monitors on Amazon for less than $100, one for only $70. The basic Duet subscription costs $4/month. So, for little more than a year's worth of that, you could have an actual monitor. I also suspect you could get a monitor for even less than $60.

If you have a HDTV set, you could try using it as a temporary monitor for the mini PC.

There are third party apps for PC/windows that offer the ability to use an iPad screen as a second screen. Not sure as the primary screen. At the very least, you’d need a PC screen to set up the settings on the PC to display the iPad. It requires 3rd party software and in some cases hardware. I’m familiar with Duet and I’ve heard iDisplay is good as well. I have not connection to either company. But I do know the chief engineer and owner of Duet is a former Apple engineer.