You need to choose an Authority to issue your licence. It can be any EASA Authority. People choose based on their own preferences of language, cost and their view of how easy or reasonably priced they think the Authority will be to deal with. Once you have chosen your Authority (known as the State Of Licence Issue(SOLI)) you get a Class 1 from medical examiners acceptable to them. From that point on you can collect modular certificates from any combination of EASA States, for instance CBIR in Hungary, MEP in Lithuania etc.. The only constraint is that all the ATPL exams have to be taken with a single EASA State, Austro is a common choice. Once you have certificates such as the CBIR pass you can send them on to your SOLI and they will add the rating to your licence. Sometimes you do not pass the certificates directly to the SOLI; in the case of the ATPL exams, for instance, when you apply for your CPL/ATPL you identify on the form which State you used and your SOLI will ask that State directly for confirmation of the exam passes. From the outside the system seems to work reasonably well, the only complaints are from the bureaucratic delays and high charges in some of the States.
As far as the astigmatism goes, although I am not an expert I believe that even though all EASA medicals should be the same there are still differences in how various states apply the rules. In this case it sounds like Switzerland would be a good choice for SOLI but others who have the same issues will undoubtedly be able to give you advice. There may be another State that would be a better SOLI overall. Once you have your Class 1 as long as you keep your licence with, say, Switzerland, you will only be examined under Swiss rules. You will not need to take a medical under any other State unless you switch your licence to have that State keep your records, a new SOLI.