The current standard, used at a minimum for obtaining the FAA repairman certificate, is the Aircraft Electronic Technician certification, through the Aircraft Electronics Association. The certification is known as the NCAAT AET (national center for aerospace and transportation technologies, aircraft electronics technician) exam. Taking the NCAAT AET and obtaining at least one of four endorsements (ANS--automatic navigation systems; OCS--onboard communications and safety systems; DNS--dependent navigation systems; RCS--radio communication systems) can provide test results, and an endorsement that is used along with an application for a repairman certification, to obtain FAA repairman certification. The FAA recognized the NCAAT AET and associated endorsement(s) as qualifying for the repairman certification. This bypasses the 18 month experience requirement of 14 CFR 61.101(a)(5)(i), with 61.101(a)(5)(2).
https://aea.net/aet/pdf/FAA%20letter...2018%20(1).pdf
The FAA still does not have a dedicated avionics technician certification; this is had through repairman, or mechanic with airframe rating certification. No maintenance can be performed unless a mechanic has training and experience in a particular operation, and an airframe rating allows a very wide range of privileges, on all categories and classes of aircraft. Short of mechanic certification with an airframe rating, one can either use a repairman certificate, or work under a repair station certificate. In the case of the latter, one does not need to hold repairman or mechanic certification, as the certification is held by the repair station.
Regardless of certification, adequate training and experience is necessary to be of value to an employer, and there are various programs around the country which offer training with an avionics specialization.
EASA certification is not necessary unless one intends to work on aircraft registered in EASA states or work in EASA programs or carriers.
The various testing and certification will serve to make one more marketable to various employers, repair stations, etc.