Hi, Kapo. I'm going to try to help you out on this one as I am in the same situation. I have a EU passport and am converting my ICAO ATPL into an EASA one. All my answers are based on the UK CAA, which I think is by far the most organized for doing this. My pesonal belief if that, even with Brexit, the UK licence will still be accepted throughout Europe and, if necessary, the conversion of a UK issued licence to another EU country should just be a matter of very simple paperwork. I do believe that the UK will still be part of EASA after Brexit, so I think it's worth trying to do the process with the CAA.
With your experience you are exempt from an ATO. You can do the studying and book the exams on your own. All you have to do is register in the UK CAA website and they'll request some documents proving that you have the requisites to be exempt from an ATO. Usually no more than a week after sending theses docs you'll be approved to book and sit your exams.
One big advantage of doing the process through the UK CAA is that you can sit your exams in different venues throughout the world and there's one available in Orlando, which would certainly make your life much easier and save you a lot of time and money.
Your theory ATPL exams are valid for 7 years from the expiry date of your IR.
After doing the skill test in the FFS you'll receive the ATPL licence with the rating on the aircraft in which the test was done. Usually it's valid for one year. If you don't get to start flying for an European carrier in the meantime you'd have to do a skill test in a FFS again after a year to keep you licence/rating current.
You can do the skill test in any approved EASA FFS in the world. One place in the US that I know you can use is Pan Am Training Center in Miami, but I believe there are other training centres also approver by EASA, you just have to check it. Doing the skill test out of the UK just requires additional paperwork, but it's quite simple.
As you live in the US you actually would just have to go to the UK once for the initial medical exam. At least in the UK the initial medical exam must be made at an accredited AME, so you'd have to go to London for the initial class 1 medical. After that you can renew your medical using approved doctors/clinics in the US.
As for the studying material, I am using the following:
- Bristol Full ATPL conversion material (CBT + Question Bank).
- Phil Croucher's EASA Professional Pilot Studies book.
I'm sorry for any typos or mispelling. I am typing from my cell phone. When I get back home I can send some links with theory reference from the UK CAA and where you can buy all the study material.
Hope this helps and let me know if you have additional questions.