Picking up Religious ideas is NOT a waste, converting Muslim provinces without it takes too long, as at most you'll only have two missioniares (1 base and 1 from DOF), both of which will be slow because you're missing that 3% extra missionary strength, and the cheaper stability + the frequent events with +1 stability are too good to pass up as someone like Russia or Vijanagar
And another one from owning Jerusalem and another one from Rome, one more from Mecca.
It is entirely possible (and quite convenient frankly) to stay without religious all game. It is the only way to go if someone wants to have permanent 100 army tradition and naval superiority over Spain/GB (hence religious is "a waste"). I converted Constantinopole without religious ideas by simply taking all the missionary strengths decisions (negative tolerance and unrest), maxing out PA and occasional hiring of inquisitor. There is no rush to convert heathens as was the case in 1.7 where revolts were spontaneous and repetitive.
Don't quote me wrong please. All in all religious is a great idea group, especially for Orthodox countries. But it's not needed to build large empire quickly because of the new rebel system. Besides it was highly desirable in 1.7 when the whole Tatar culture was under the same culture and these lands had a number of very rich provinces (Samara, Crimea, Kharkov, Buzuluk, etc.), which were both desirable and difficult to convert. Russia was rewarded by eating all this by accepting Tatars quickly and receiving all these lands with no malus. Not the case anymore. There is little incentive to go after hordes before PLC/Sweden are taken down completely because these have sweet same cultural group/ religion lands or Finnish lands which could be fed to the orthodox vassal.
Tatar lands are pretty poor these days and could be best acquired through vassalisation, which doesn't result nationalism. In additiona you are free to use vassal horde's awesome CBs.