I had hope that your project will be not THAT big, but it is even bigger.
My biggest worry about big projects is that as indie you will never finish it in time.
Now let me explain:
Project like this requires years of man labor work to be done.
Hard truth is that you need to finish it before njext generation fo graphics, hardware and software comes out. I would say that it is 5 years for next gen, and about 10 years for next next gen. So in 5 years game will be just fine (Average of all other games), in 10 years it will be “a bit outdated”. I seen this happened to project i joined, those folks did second job after real job just to finish it, it was their dream, they did everything they could and all that was just barely enough. And scope of their game was 1/10th of yours.
That is why i wrote “doable in one year” because i know there are ALWAYS delays. With this you are aiming at 10 years at best. Just look how long Cyberpuk (or Mass Effect Andromeda) were in development and how many people they had. Where you put single programmer in they had teams or 20 or more people. You cannot do more with less people, they all are already milked beyond any sane limits in those AAA corporations.
Just minimize scope of your game, then minimize again. Think about current (after you minimize everything) game as research project and aim to accumulate some funding, for real game after you finish this one. You can also introduce new trademark/setting story to market. Going all out on first project probably will just end in disaster.
ps.
for anything “massively online” you need team of few engineers to setup backend services and hardware, also few programmers (that are not unreal, but C++/python/golang etc.) to develop server side code and functionality), also some $$$ for actual servers.