Solution For Trojan Virus That Doesn't Remove After A Factory Reset?

I recently purchased a cheap Android 8.1 X21 phone, discovered it has some malware installed as part of the system files. It continues to install random apps and displays annoying pop-ups. After a factory reset, the malware still exists. Malware Bytes found the virus initially and couldn't remove it. The two system files that were identified as malware are FWUpgrade and FotaProvider. I am prepared to learn how to root my phone; any other options?