I have a Kimber .22 conversion slide for my TLEII. I've put in a 18# hammer spring which helps the pistol cycle using subsonic ammo (I usually shoot this with a SiCo Spectre II). I still have numerous failures to feed, particularly with the first and last round, and the only thing I can point to are the crappy plastic Kimber magazines. The barrel ramp is polished nice and shiny.

Who makes a compatible metal magazine, or one that's proven to be more reliable?

I have some Ciener magazines that work well with my Kimber conversion.

I have also used Ciener magazines with a kimber 22 conversion. In my experience they worked well, but not better than the kimber nylon mags.

Sounds like you are doing everything right with an 18lb mainspring, sometimes the kimber kits just arent that reliable compared to say a dedicated rimfire pistol like a browning buckmarks or a ruger MK etc.

Check your breechface on the kimber slide. The entire slide is made of soft aluminum and after a while the breechface can develop depressions that will affect feeding. Also the slide rails can wear allowing vertical play which can also affect reliability.

If anything the gun is TOO tight. I'm going to try to break it in manually.

I did order a GSG metal mag, we'll see if that improves things. I do agree that it's tougher to make the full size 1911 reliable compared to a dedicated .22.

If the slide is too tight, you can take one of those dremel wire wheels with some oil and burnish the slide rails to smooth out the paint in there. Also clean out the recoil spring plug as they sometimes are filled with sand / grit.

Run the kit with a light viscosity oil and keep your fingers away from the slide when shooting so you don't induce drag.

It can’t hurt to polish the chamber.  I use Flitz polishing compound on a barrel mop attached to a short cleaning rod spun by a hand drill.  CMMG recommends buzzing the chambers of conversion kits and barrels with a brass brush with gun oil spun by a hand drill.   I’ve found polishing the chamber really helps function of 22 rimfires.

You may have a hard time getting in the chamber with a mop / cleaning rod as the staked ejector is partially in the path of the chamber.

I have an Advantage Arms 1911 Standard kit and an Italian knock off kit where they have the same style of ejector attached to the barrel.  You can polish the chamber more easily using the mop from the muzzle and using care you won’t damage the crown.   It isn’t as easy as other rimfire barrels but polishing the chamber on blow back 22 rimfire has greatly increased reliability for me.

Well the GSG mag didn't have any positive effect. They are easier to load though.

For some reason, those first two rounds just don't want to hit the ramp at the right angle, but rounds 3-10 are fine. I'm going to do more testing this week (like load 8 rounds). I polished the ramp to mirror brightness with Mother's and a Dremel so we'll see if that changes things.

Extraction and ejection are very good, it's just the geometry between the magazines/cartridges/ramp.