An ICAO PPL is enough to enrol in the ATPL theory and CPL courses. Points B.1. & E.2. in appendix 3 to Part-FCL.
A foreign licence validation may be used to gain outstanding PIC experience, if the LBA will issue one. Point (3) of article 8 in Commission Delegated Regulation (EU) 2020/723. AOPA Germany provides an overview of this option
here. Alternatively, German law might automatically render third-country licences valid for
Annex 1 aircraft registered there.
The revision weeks for the ATPL theory course can be done online, assuming it's offered by the training organisation. These are intensive periods of knowledge consolidation intended to condition you for passing the exams. Your physical location while attending online is immaterial. The thirteen subjects and associated exams will be divided into modules. Austrocontrol has exam centres in numerous European countries, in addition to Port Alfred, South Africa, Naples and Fort Pierce Florida, Portland Oregon (Hillsboro Aero Academy, soon to be an EASA ATO), and in São Paulo, Brazil. There are bound to be more outside Europe under other EASA authorities, eg there's one under the Romanian CAA in Johannesburg, and another under Transport Malta in Sebastian Florida.
Transfer of licence and medical records is typically done as a condition of employment. Do yourself a favour and pick the initial competent authority based on positive feedback from licence holders. Examples of transfer fees are discussed in this post,
UK issued EASA licence after 31st Dec 2020
The AZF E will be needed to control a German aircraft radio station. While under dual instruction, that role will fall to the instructor, making it unnecessary for you to hold a German certificate. Bundesnetzagentur can tell you whether a foreign radio telephone operator certificate is acceptable.
If you select Germany as the state of licence issue, you'll need to complete a background check, called a ZÜP (Zuverlässigkeitsüberprüfung). See
Luftsicherheitsgesetz (Aviation Security Act), § 7.
The KSA module and its method of assessment is explained by one of the leading theory providers
here. For the exams you'll need an ARC-2 or CRP-5 flight computer and an electronic calculator such as a Casio FX 83 or 85.