They are already selling data to midjourney, and it's very likely your work is already being used to train their models because you have to OPT OUT of this, not opt in. Very scummy of them to roll this out unannounced.
I drew for myself a sheet of stickers with Oscar from the game Syberia. I love him very much, but I couldn’t find any merch so, as they say, if you want something done well, do it yourself.