This would be simple for the user to execute but it would very quickly be spotted by the operator as it's all from a single originating MSISDN.
Spreading the load over many users like your latter example would be a lot harder to spot, as would spamming through multiple SMS providers as you're diluting it (but it might also get picked up by the provider e.g. Twilio, MessageBird etc).
My point was that most spam originates from people with SS7 access and not SIM cards. It can also come through low cost SMS providers but is short lived as it's blocked the moment it's discovered or there's a complaint.
