I'm on board with this. I think both routes are good.
My understanding (I'm not a lawyer, THIS IS NOT LEGAL ADVICE) is that this may break article 102 TEFU because the dominant undertakings (Visa, Mastercard, etc.) are "refusing to supply input indispensable for competition in an ancillary market", in this case supply input is providing payment processing services and ancillary (subordinate) market is digital distribution of software (Steam, Itch and their competitors). However I couldn't find similar precedents.
I think the citizens' initiative may take longer because it needs time to collect the signatures before any procedure can even start, but I may be wrong on this. In any case I think getting help from Stop Killing Games is not a bad idea.