Now I’ve read that my (frozen) ATPL is valid until 7 years after my last IR check. Can somebody confirm?
You may have read this in EASA Part FCL, as it is correct.
FCL.025
(2) The completion of the airline transport pilot licence (ATPL) theoretical knowledge
examinations will remain valid for the issue of an ATPL for a period of 7 years from the last
validity date of:
(i) an IR entered in the licence; or
I certainly don’t want to lose my ATPL
You don't have an ATPL.
Or can I just get my SEP rating again. Try to get some MEP hours afterwards and contact an examiner myself?
How will you get MEP hours with an SEP rating?
The exams will remain valid for the issue of an ATPL for 7 years from the last IR.
Everything is delegated to ATOs these days, so find an ATO (Probably the one you used for the initial) and they will come up with a training plan tailored to getting your IR back up and running, complete the training and pass the test and that's you golden for another 7 years. As your rating has expired, you would not be able to present yourself for test to an examiner without a course completion certificate stating that training as required by the ATO has been completed.