Welcome to the other side of the pond! I just finished my EASA conversion this past summer. Bristol Ground School and their digital learning program is a good option. I used that combined with a question bank to prepare for the ATPL theory exams. It worked very well.

If you have at least 500 hours in one of those airplanes (737, RJ), then you can be exempt from a formal training course and self certify for the ATPL theory exams. But I would still recommend at least buying a home study course as the questions and theory can be quite difficult. Just memorising the questions and answers is not going to cut it, and you would have to memorise around 15,000 questions.

A few other things to be aware of-

The UK has defined the conversion process very well and I would recommend doing at least the ATPL theory exams through the UK. The UK has an exam center in Orlando, not far from Universal Studios/International Drive, so you do not have to fly to Europe to do the exams.

Your initial EASA medical exam will have to be done in Europe, and your license will be issued through the country which holds your medical records. This country does not have to be the same country where you do the theory exams. For example you could do the 14 theory exams in the UK (all 14 have do be done in one country, no doing 7 in one country and 7 in a second), then get your medical done in Ireland. Your skills test can be done with any EASA examiner and in EASA country, but the actual license application will be sent to the country which holds your records.

Some countries are very easy to deal with, and others are much harder. Ireland has a very good reputation as being easy to work with and being quick to issue licenses. Spain, Portugal and Italy all have a reputation for being very slow and difficult. On occasion some of the southern European countries will not accept skills tests from outside their country. Generally speaking the UK, Ireland and Iceland have good reputations. Sweden and Denmark do as well, but their fees can be expensive.

If you choose to have your license issued in Germany, German privacy laws prevent releasing your medical records to other countries. Officially any EASA license can operate any EASA registered aircraft, so a pilot with an Irish ATPL can operate a French registered aircraft, so most of the time state of license issue is not a problem. But a few airlines have some restrictions on state of license issue. For example easyJet requires you to have a UK EASA ATPL, and SAS requires your license to be Danish, Swedish or Norwegian. If you have a German EASA ATPL, it be difficult to change the state of license issue due to their medical privacy laws.

Many nations require a radiotelephony license as well, and getting information about it can be difficult. That license is often left out when researching the conversion process. I did the theory exams, medical and skills test, only to find out after all of it that my CAA required the radiotelephony license on top of it all. I do know Ireland does not require the separate radio license, and because of that they may be they way to go.

Lastly, if you are going to live and work in Europe, be careful about having your license issued in the country in which you reside. It is not uncommon for tax authorities to get access to the national CAA pilot database an single out pilots for extra tax scrutiny. For an extra layer of safety, if possible, have your license issued in a different country.

Hope that helps. Feel free to send me a PM if you desire more information.