As someone who basically lives and breathes communist flags (left-wing flags are seriously my main hobby) and mods on a sub basically dedicated to them, I think I have the authority to tell you that... You're absolutely right. Maybe more than you'd think.

Communist flags are an exception flag-wise in that the real ones tend to look better than the fictional ones. You mentioned the "historical precedent for red/gold and the hammer and sickle" and, while that's true on the color part, it's really not on the symbol part, because the hammer and sickle was effectively the national symbol of the USSR, and you don't want someone else's national symbol on your flag. The flag of China and the flag of Angola are great because they both specifically refference the soviet design without using a hammer and sickle (the flag of angola is even derived from the viet-cong, another communist movement)

Red flags are basically the oldest symbol of socialism, so the color red on a flag is something that has to stay. However, that doesn't mean your flag has to just be a USSR wannabe. Look at mozambique for example, with the cuba-like red triangle.

And that's not into getting into non-national communist flags. My point is that communist symbolism is a very rich subject that works very well with flags as a medium, and for people making fictional flags it's usually very under-explored. This sub is the worse one at that, if you want some slightly better designs browse r/leftistvexillology sometime, but even there most of the flags suck. Some of them are great though, and really showcase the potential of that kind of symbolism on a flag