Step 8 is problematic. (I would also quibble with wording on others, including 2, but those are less critical.)
What exactly does “real” mean there?
11 is also problematic. At best you can get that a model which assumes other people are conscious similar to you will help your predictions be more accurate. This says nothing about whether other people actually are conscious.
You can’t actually bootstrap a refutation of solipsism.
I would claim that ultimately anti solipsism is a moral position, not a factual one. I.e. it is a moral good to treat others as conscious. I can’t write up a full argument for that though.
I disagree with this, and strongly disagree with your claim that it is obvious.
You are assuming that sentience/ability to suffer is a factual question, when it’s actually a moral question and it’s perfectly fine to apply moral evidence to moral questions.
The exact argument depends on definition. If you define suffering as only applicable to moral patients, then the question if something is suffering requires a determination of whether it is a moral patient , which is there a moral question. If you define suffering more broadly, then whether it is bad depends on whether it’s happening to a moral patient, which again is a moral question.