Well then I guess you should try 6 then! At least then you will know if the problem is UT, right?
If you get a 5 am WU it will suck horribly but then you probably will know that you are dealing with OT and not UT causing OT...
Have you tried days that are shorter than12 hrs? Like 11 or 11.5?? I know that seems insanely short and that he never tacks but maybe you will be surprised? Do you really think that if he goes to bed at 5 he will wake at 4 for the day? Mine never tacked until the last 2 months or so but even so If I would do 5pm BT he might wake at 4 but eventually, it may even take an hour and a half of WIWO I can get him to fall back asleep for another sleep cycle or two. It is torture when doing it but at least WU is much later AND you got more sleep into him, yk? It has never happened where we got a 4am WU and we started our day, NEVER! If I stick with WIWO he falls asleep, even if it is just out of boredom, it always seems to work!