I have long been a fan of reading the original which in this case is a very short article. Here is the original in English:
https://www.fourmilab.ch/etexts/einstein/E_mc2/www/
Another good read of the original is James Clerk Maxwell's Treatise on Electricity and Magnetism where he supposes light speed is always a constant (about 300,000,000 meters/sec). JCM just "supposes" this. He also supposes that light is just electromagnetic radiation after looking at the sameness of the equations he developed for measuring electricity and those for magnetism (and tweaking them a bit).
Most people don't know Einstein grew up in a family that was electrifying Bavaria. He grew up with Maxwell's equations in his life. See Isaacson's Einstein.
https://www.amazon.com/Einstein-Walter-Isaacson-audiobook/dp/B000PAU1UO/
And for my two cents a lot of these thought experiments are just ways to imperfectly describe things we do not really understand, whereas the math never lies.
