That is really only a problem for the new class mods and M4 only weapons that were added with the Maliwan update. Pretty much everything else world-drops in which case you can farm it from virtually any boss.
The thing to know and keep in mind with the dedicated drops is that you have the exact same odds of getting one no matter what level you’re playing at. So, your odds of getting a dedicated item drop is the exact same with no Mayhem as it is at Mayhem 4. Your odds of getting an anointment increase by increasing Mayhem levels, but going higher levels in Mayhem does not give you better odds of getting the item to drop.
I’m doing some testing and am almost done. Based on my preliminary data if you’re farming for anything other than the M4 only items then you’re best off farming at Mayhem 2 as it’s the best balance between difficulty and time to complete the farm and the amount and type of loot. Mayhem 3 only has about an 18% increase in loot drops over M2, but can take significantly longer to complete. Over the course of time you could farm more gear making more of the faster M2 runs than getting the higher average drops per run of M3.
M4 farming should only be done if you’re looking for the M4 only gear. So far it’s looking like an 80% increase in loot and number of anointments over M3, but the increased time and difficulty means you could probably run more than twice as many M3 runs in the same amount of time.