This may not be a big, but you can not build litematica schematics even though schematics is 1.12.2 and older version.

.litematic schematics are not compatible you could use worldeddit to make one schematic .shem that is compatible but that can be annoying
Had the same problem and was looking for a lot of different approaches the best one i got:
i have found a program on github that converts these .litematic files to .shem files

https://github.com/GoldenDelicios/Lite2Edit

are you using baritone for survival on a public server?
bc there are already many good features in litmatica to place it in the world like different printer modes