LED bulbs in my old house are dying prematurely
Cheap $$ bulbs

This is a quality problem.  The market is flooded with boatloads of cheap Cheese junk, and "thrifty" buyers gobble them up.  

These manifest on ebay and the dollar store, of course, but they also show up in bigger retailers.  And just as WD-40 made a brand out of missile juice, many junk importers make a brand simply by being everywhere. You see Feit Electric so often, in so many shops, that it must surely be a top tier product, right?  Nope.  Not at all. It's junk. Same for Utilitech and Lights of America.

Generally for quality lights you can't go wrong with Cree or the old Pheobus Cartel members, particularly GE, the only one who still makes their own LED bulbs. The big makers consider screw-in LEDs to be a dead-end business, since once everyone has fit them, they won't be able to sell any since they don't fail.  Obviously, the dollar vendors have figured out how to solve that problem!

Lost Neutral

This is improbable, but very serious, and I think of it because of your lights flickering.  Its symptoms are rapid, inexplicable failure of appliances. 

It may be that your house's neutral wire is damaged, probably coming in off the pole, with about a 95% chance the problem is on the power company's side, and thus, fixed for free.

You detect this by measuring your voltage on a variety of outlets. It indicates the problem if some of them are below 120V, while others are above 120V by the same amount.  Your house has two 120V "legs" of power.  For instance, my house read 94 volts and 144 volts respectively, the two values added up to about 240V. Equipment on the high-voltage side is prone to damage. These values will "teeter-totter" back and forth depending on how much load is on each leg, with a new load causing quite a change in voltage, and thus, "flickering".  

LEDs should ride right through small changes in voltage. 

Enclosed fixtures

LEDs need to run cool, and they dislike heat. The LED itself needs to run cooler than 85 degrees C (185 F). The electronic driver circuit is also not a big fan of heat, especially if it's built cheaply.  The problem is, old fixtures are built for incandescent lights, which love heat. So fixtures aren't built to remove heat from the bulb, they're designed to bottle up the incandscent's heat to protect the building from it.  Given those design factors, enclosed fixtures make sense.  Not with LED.