Well guys I played probably 50 more years since the OP and I took over most of the ERE, usurped the title (with the heir of the first Emir, so only on my 2nd ruler here) and then my ruler died of old age a few years later and I took over as the 3rd ruler. Just 2 years or so of cleaning up various rebelling Sheikhs and I was good to go. No succession crisises or anything like that. Decadence is at 19% and +0.19%. Honestly I don't see what the big deal is, I have 4 or less people generating decadence at any time and I try to keep all adults in jail, giving out titles to underaged people (my own sons preferrable since I can't jail them for free), if you do this fast enough you can even catch them before they have any sons and when they die in prison the lands just go back to you.. Rinse and repeat to reduce your decadence rating.
Plus not to mention all the events you can do like Ramadan or whatnot to lose a bunch of decadence (that's how I dipped below 20%) for really cheap once you have a whole empire at your feet.
Heck the only big mistake I made all game is accidentally letting the wrong culture son become the new Emperor (Maghreb instead of Bedouin) which made a few more people rebel than necessary since I installed all Bedouin counts. Next heir I'm going back to Bedouin (married all my wives to be genius Bedouin and one Midas Touched descending from Muhammed as a wildcard, but unfortunately not a genius). Have like 20 titles in reserve I can create and give out, but most of the counts are vassals directly to me.
Honestly I don't see how this game isn't trivially over at this point, I can take over the western Muslims easily (and the Egyptians won't care since I'm allied with them, or I can just wait for a war where I side with them to be doubly safe) and then gobble up the smaller Christian kingdoms. On the eastern side I probably just want the lands north of the ERE to let the TGH have it, maybe they'll even fight the HRE for me if they get that far...
The only thing I can say about Holy War is that its usefulness seems to diminish later since as far as I can tell it destroys all buildings within a province, so it takes forever to build new ones. Great for getting a bunch of piety, not so great for getting a bunch more levy.
The real annoyance here is that every time I raise levy/fleet I have to spend forever clicking on each one to embark them onto a fleet. Unless there's some kind of auto raise and embark option I'm missing, I think that alone is going to make me stop playing very soon. Too tedious... too tedious.
Decadence management isn't hard, just tedious, you can have loads of landed dynasty members and be fine as long as you keep conquering. You can only run into major issues
once you rule the world as you dynasty continues to grow and you have no new land to give, or battles/sieges to fight. Giving newly conquered counties, 1 each to three under aged cousins is fun though, usually one will die before maturity, especially if they have different mothers.
The simplest fix seems to be don't let any religion HW CB vassals who are allies in a SC. It would be ok to HW the primary participants or CB to press claims, but I've used this before to pick off Byzantines especially, things like border duchies who join the claimant to the throne as allies such as Armenia are easy to pick off for external powers, or 1-2 county allies of a claimant. Alternatively for a SC, make all participants join their ally an make him their liege. Thus instead of 20 factions in ERE in a SC, you have the owner of the throne, and 2 claimants at war with him and each other. It's much harder to win a HW vs someone w/ 1/4-1/2 of the empire than it is to conquer to counties and force a surrender at 100%.
Huh? I don't think I really understood that with all the acronyms being through around. SC = succession crisis? In my OP I said I holy warred all the duchies of ERE in a succession crisis (except the one who called the war), isn't that the same from what you're suggesting?