I advise younger throwers to throw in the fall then take a month off in November to let the arm rest. The experienced thrower needs more time off, plus my last meets are in mid-September, about four months later than most others. Weighted javelin throwing is used extensively by the Germans, Sergey Makarov, and other top throwers. I don't advocate letting rip on a heavy javelin early, so be careful. Keep your high efforts to a 900gm javelin.
Heavy javelins are actually a superb tool for development.
I use a heavy javelin early in the training and just throw smooth, a easy throws to condition the little muscles in the arm to withstand the pressures of full bore efforts later. It also helps you develop arm power most conducive to javelin throwing. This is the critical time a good throwing athlete makes their career. The birth of a javelin arm. I like to focus on the javelin landing flat, or throwing low line shots.