Most phones can't be unlocked with ADB/fastboot. And even if they can it usually factory resets the phone.
Anyway most people don't want to root, don't want to have to put in the time to learn enough to maintain the device afterwards, and don't want to have to deal with apps that won't work on rooted devices. I've rooted a lot of phones, and I've modified them far more deeply than just flashing a ROM, but I'd not recommend it unless someone has shown that they are interested and willing to put in the work. For a general user problem it would be my last resort, not my first.
Just out of interest
@Houzvicka , what model of phone is this? There have been some models, generally intended for the Chinese market, which have adware as part of the system software. That
shouldn't happen with a major brand, but there have been cases where reputable manufacturers have included apps as part of their system software and then the app developer has pushed out an adware update to that app (the the version that came with the phone was fine, but after the update it began pushing intrusive ads). "Peel Smart Remote" was one example of this. So it might be worth looking at the pre-installed apps as well, and try disabling or uninstalling updates for any that you aren't absolutely certain of.