The email "I'm afraid we have no choice but to sue for defamation" is pretty nonsensical. Of course they have a choice. If they understand the law, as I would hope that they do, they would understand that a true belief/ opinion is a defence. From your account they did do a poor job, were rude and unprofessional - nothing you said was untrue.
They also have to demonstrate 'serious harm' to their reputation. One disgruntled client publishing a review that was taken down after 2 hours is not going to do this.
I would suggest that at this stage, they have not issued any form of claim against you. There is no letter before action, there are no court papers issued.
My view is that I would be very surprised if this went further.
I'd also suggest that you could complain about the level/ quality of service to the solicitor's regulation authority. This would strengthen your case that you held a true belief if a complaint is held up by the SRA about their poor service!
I find the tone of his email very unprofessional "I will not reply any further as you are now wasting my time..."
If I treated clients like that I would have no clients at all.
I hope that this assists?