Ive reset numerous times...I was in the same mindset as yourself, I had an s8 when they first come out and had it for 3 years and loved it! I upgraded to the s10, didn't insure it and destroyed it with one drop. so I ended up getting an s8 from my brothers boss who smashed all the back but the front is untouched...I am now using that phone and the reason I wanted to flash the stock ROM is because It is believed his partner may have tried to use a spyware app of sort to keep tabs on him... I didn't trust just a partition wipe and factory reset so opted for a fresh install. Now, ever since looking into the possibility of spy apps etc, I've come across numerous examples of an S8 being remotely infiltrated without any sign or notification to the user that this is happening... of course that is only possible if you have access to the device itself to installed the malicious software... so, if that is the case with the phone I'm using, is it possible that it could permanently of embedded itself to the system? Certain actions and apps act out of character from time to time, this is what raises my concerns as I had one for a couple of years and never had any problems at all... something just doesnt feel right.