Instead of only having the ability to mine ores, have the ability to mine a specific block given, i.e: "#mine amethyst_block"

already possible. did you not even check
on 1.12 it's #mine log
on 1.13+ it's #mine oak_log

already possible. did you not even check on 1.12 it's #mine log on 1.13+ it's #mine oak_log

I tried it on meteor client, but it didn't work. Maybe outdated version of baritone ig?

if there's none of the ore in range it stops. use goto in that case

MineProcess will also explore if legitMine is enabled and on more recent versions it will behave like GetToBlockProcess if legitMine is disabled (i.e. explore if exploreForBlocks is enabled, fail if it's not).
The versions in which non legit mining can explore are v1.8.2, v1.7.2 and you can build it yourself for 1.12.2 from master.