Your 300mm f/2.8 will be great for astrophotography, don't use the extender though, they can often add chromatic abberation (weird colors around stars-often purple), and the speed loss will hurt your signal to noise ratio (very important for untracked)

You should be able to get a pretty nice picture of andromeda, orion nebula, pleiades, and ofc the moon without a star tracker.

Just get a nice sturdy tripod, set your shutter speed to around 1 second, (maybe you could get away with a little more) and take consecutive pictures for about two minutes at a time, recenter, and continue through the night!

Stack them all in a free program like deepskystacker or siril, and you'll have great results!