Is it possible to enter directly as a tax specialist? I will be ACA and CTA qualified at the point of entry

Yes, it is possible. HMRC advertised for tax specialist jobs in the last couple of months (over 70 at least) that were direct tax specialist positions and CTA was one of the accepted qualifications. My advice would be to keep looking for these on CS website and set up an email alert for the criteria. In the meantime, you can start looking at success profile behaviours and prepare some examples. You will also be eligible for accountant job in HMRC and again these come up sometimes.

Lubna has answered your questions beautifully and I'm just going to add comments regarding the additional information provided. Regarding pay cut, being tax qualified is a highly valued skill and whilst HMRC is one of the higher paying departments it does not compete with the private sector (once experienced). As you will be recently qualified, the pay might be comparable (basing on average, I don't at which end of the sector you work in) but long-term the private sector has significantly higher ceilings on what you can earn compared to a career in the civil service (but you have better job security and a defined benefit pension that rarely exists in the private sector). Money isn't everything and there are other things that HMRC offer you can't find elsewhere but given how early you are in your career, I would recommend speaking to someone has been both in the private sector and HMRC to help you understand where your career aspirations and values best align (and this can change over time, you will have 30 years of work ahead of you so plenty of scope to change).