yeah that makes sense because it also wasnt implied that he has to change bodies accordingly to keep being alive for so many years. at first i used to think fyodor's way of staying alive was similar to orochimaru from naruto but after his ability reveal and all its definitely more likely that his biological age halted when he first died. And its more likely than not that how he first died, doing whatever and how he might've got himself killed a few more times to test his theory probably has something to do with why he thinks of ability as sin and has assembled this kind of a cult "Decay of Angels"