I think 100mm macro makes sense to use it on APS-C I think it doesnt matter much what you buy first. Im also thinking if i should get an L lens before getting a FF body I can only buy one. So i can either start FF with 50mm 1.8 II or get some L (24mm 1.4 for example) and use it on my APS-C now already.
Soon you will be singing the Trainspotting song which also applies to L lenses:
My therapist said, not to see you no more
You're like a disease, without any cure
I am so obsessed, I'm becoming a bore
Oh, no.
And you think you're so pretty!
That was also my first L and I love it! Great choice! After I saw the IQ from that it made me realize why L lenses were so sought after! Now for my first L prime!!