There is a new video up showing this interaction in CK3 Royal Court.
Just a few thoughts regarding Mystical Ancestors PRIOR to when Royal Court was dropped.
Now, this wasn't the exact strategy I used for getting all the renown in my All Legacies in 867 Run however it is quite similar and most importantly easily reproducible even in Royal Court!!
So by paying a bit of prestige you can gain a lot of renown.
Now currently in Royal Court Mystical Ancestors gives 50 Renown per granted County, 75 per granted Duchy, and 150 per granted Kingdom. (In fact 250 per Empire, but I personally would never do this as the strat will no longer work)
Furthermore, keep in mind this strategy only works if your character's heir also has you as his heir. So if you have 2 sons, it doesn't work as well.
