Hi,
how does one stop blobs like Abbasid from imploding due to internal strife?
1. Reduce decadence to zero and keep it there. Contrary to popular belief decadence is not super-difficult to control, you'll just have to a) keep good pious traits yourself and b) occasionally lock up decadent family members. The latter is easier if the family members aren't landed, but landing them reduces their chance of becoming decadent.
2. Optimise power structure. As a Muslim empire the Abbasids have a massive advantage over others as you can revoke duchies without incurring tyranny, and you can also freely retract county-tier vassals. Both of those still make the target lord angry, but nobody else will care.
Optimising vassal power structures is basically the main point of CKII and everyone has different opinions on what the best option is. In most circumstances you want to keep your vassals weak and powerless in order to stop them from causing trouble, and the most common way to do that is to give out one county per vassal and then give out one duchy per de-jure duchy, whilst keeping any kingdom-tier titles for yourself (or not creating them in the first place). This works well even in large realms, but the vassal limit eventually starts making things more difficult for very large blobs. You can counteract that by creating 'superdukes' (the colloquial name for dukes with more than one duchy), but they tend to become power-hungry and dangerous. As you keep expanding you'll end up with more and more superdukes and you will not be able to control them all.
For very large empires like the Abbasids a different strategy is better. Rather than keeping control by weakening your vassals, it can be more useful to use personal diplomacy to keep them in line: gifts, marriages, hostages, council positions, feasts and non-aggression pacts. This approach works better if you minimise the number of vassals rather than minimising their size, and that means creating enormous vassals far larger than any superduke. If you can divide your empire between five or six 'hyperdukes' or vassal kings you can keep an eye on all of them. In fact, if you have fewer than seven extremely powerful vassals you can more or less completely eliminate the threat of civil war by empowering the council and giving them the right to vote on war declaration.
The other advantage of this strategy is that it's popular. Revoking titles makes people very mad, especially revoking counties as that will still cause tyranny. If you want to reduce all your vassals to one county each you'll end up having to fight a lot of wars and generate a lot of claimants. But giving out titles makes people happy! Occasionally things will need tweaking by retracting the odd vassal here and there, but mostly your subjects will be prosperous and grateful.
The disadvantages of this strategy mainly come from the lack of control you have over your vassals. Vassal counts rarely do anything of note, and weak dukes are usually too busy fighting their own vassals to become really ambitious, but vassal kings have the resources to really get out there and start interfering with things. Vassal kings often declare war on neighbouring independent states at inconvenient times, sometimes even launching their own private holy wars. Making king-tier viceroys helps with this somewhat as they can't establish a multi-generational power base as easily, but that requires advanced technology and won't help if they start adding counties or duchies to their own personal holdings. Vassal kings are also hard to fight compared to weak dukes. Although in a full civil war they're at a disadvantage since they're geographically-compact rather than spread all over the place, if you want to revoke a single county from a single vassal you've got a hell of a fight ahead of you. Best to avoid that kind of conflict.
3. Use realm peace as much as possible.