There is a huge difference between installs and bench work.
What avionics do you want to install? A great deal of install data is shrouded in secrecy and difficult to acquire from "the dealer network" unless you have an experimental amature built airplane in which case you don't need an A&P to begin with.
Transponders require recurring tests only a Certified Repair Station can perform.
If you want to learn a great deal for free, google "GTN-650 install manual" and study it, specifically the tooling sections, wire prep, connector assembly and interface drawings.
Repairing aircraft radios and instruments continues to get more and more specialized so much so that there really isn't any point to studying and investing equipment to do it unless you intend to open a business. Even then industry trend is is "pull the busted radio and send to manufacturer".
Avionics installs uses many disciplines, sheet metal, composites, wiring, analysis etc. Where I work there may be 800 man hours of sheet metal work in a panel makeover.
Bench is a much narrower, highly specialized discipline that has very little to do with installs.
Food for thought, several manufacturers won't honor warranty unless the radio was installed in a certified airplane by a dealer. In homebuilts, the basic harness is typically supplied by the dealer.