Should I replace my disk brake calipers? I have a pair of Shimano Deore BR-M615s on my mountain bike with about 3,000 miles and 5 years on them. Lately I've noticed that both brakes have too long of &quot;pull&quot; before providing braking. That is, the distance from when I start to pull the lever to when the pads engage feels too long, perhaps 3/4s of an inch or so. The braking power after the pads are applied seems fine. I think the pads and rotors are within spec.
When I remove the wheel and pads, &quot;exercise&quot; the pistons, and clean/lube with mineral oil, the pistons are quite finicky. Sometimes only one moves, other times after some the fidgeting, only the other piston will move. One piston does not return all the way back into the caliper (perhaps 1/2 millimeter protrudes). Sometimes the pistons are cock-eyed, extending from the caliper at an angle. Sometimes I notice air bubbles in the lube oil between the pistons and the caliper. It's difficult to get both pistons extending evenly from the caliper. After some fidgeting, I can improve the &quot;pull&quot; distance, but as the pads wear I have to fidget more.
I am considering replacing the calipers with the BR-M6000, but wanted some input before pulling the trigger. What makes the most sense to try next? Overhaul the current brakes? New calipers? New levers, hoses, and calipers? Live with the fidgeting?
What you describe in the first part of your question is normal. Longer lever travel may indicate you need to bleed the brakes. Uneven caliper piston movement is caused by contamination of the piston seals with dirt.  Park Tool has a video about cleaning pistons and seals.
However this is bit more worrying:

Sometimes the pistons are cock-eyed, extending from the caliper at an angle. Sometimes I notice air bubbles in the lube oil between the pistons and the caliper.

The pistons should not be able to extend at an angle. That's not 'lube oil' between the pistons and caliper, it's the hydraulic fluid (mineral oil in Shimano's case) and you should not be seeing any of it getting past the seals (or leaking anywhere).
This makes me suspect that the seals are worn out. The Park Tool video I linked above shows how they work.
Seems to me that it should be possible to replace the seals in hydraulic brake calipers, but I've never heard of anyone doing it - someone will maybe add a comment about that.
I would not assume that calipers and levers from different groupsets are compatible. I tried looking this up in Shimano's compatibility information but inter-groupset caliper and lever compatibility is not listed for MTB groupsets.