Honestly, this is already a very strong essay and I’d be happy to put it at around 12-13/16. The AO1 is solid, the main improvement there would just be smoother flow so it reads more cohesively. A short conclusion at the end of the essay, giving an overall judgement would also help boost your marks.
For the AO3 point they are pretty well written and stick to a clear structure, theres a good range of points and each one has a counter argument. One thing I would say for improvements is, you've repeated phrases like “this decreases external validity” or “this maintains credit”. These phrases are good but when they’re repeated 3 times it makes your paragraphs sound a bit formulaic, maybe tweak the wording a bit. Also external validity is not always relevant, for example in your second AO3 paragraph, it would be more appropriate to reference ethics as a link rather than external validity.
With those small tweaks, this could very realistically reach the top band.
If you have any other questions about this essay or 16-markers in general, feel free to ask, and if you’d ever like structured 1-to-1 help with Psych, I’m happy to help with that too.