I also got mine printed through jlcpcb. Since they also are load bearing I used PA12-HP Nylon (MJF). After a couple of years already in use, they didn't deform or degrade.
Concerning finish: you can let them sand for a smoother and get them dyed in a color of preference. You can also do it yourself with some fine sandpaper and a simple spray can.

Yep I think the way to go is prototype in whatever you have lying around, then order a final print in the strongest material that makes $en$e. These tiny pieces should be dirt cheap.

Nylon filled with glass beads is often recommended when stiffness is a concern; JLC offers FS3401GB (Farsoon 3401 glass bead) Nylon in this space (I'm guessing it's similar to HP's glass bead nylon (HP 3D HR PA 12 GB)).

https://jlc3dp.com/help/article/3401gb-nylon

Colorwise it doesn't match most bicycle components though as it's a lighter grey; some additional time with the paint brush is likely required.

i have made custom spacers for my 3T exploro. i have used petg CF to print it.
No problem after one year. di not touch the setup of the headset.

I have also printed some spacers for my focus izalco max from 2018 which now uses a FSA SMR 2 stem. I printed the spacers in ASA which worked quite well even though i "only" have an Bambulab A1 mini. I did that around 1 year ago and have not experienced any problems. I am a heavier rider at around 80-85kg. I must admit that i have done it solely for the looks and no other reason