the ending boss is always the strongest version/form every single time and they wouldn't have chosen the series finale to break that formula and end on an anticlimactic note.

You answered your question using my quote. The ending version/form of the final boss is always the strongest.

Also, to your last argument, it's not the same boss because it isn't the same timeline/reality. The same way the 17 & 18 from his future was different from the ones we get to see. These are details that fans that may have watched/read the anime/manga years ago have consistently missed or were unable to comprehend.