Does installing a custom ROM solve spyware problems on a Xiaomi?

I have done extensive research into the world of ROMs, but could not find the answer to the question: whether a device contains spyware on the firmware level. I want to know whether unlocking the bootloader, and installing a custom de-googled ROM solves the problem of spyware on a Xiaomi phone. I'd like to point the question towards Xiaomi, but in general, it would be nice to hear something on the overarching issue of: does installing a custom ROM on a mobile device solve spyware problems?