The problem with a lot of these dream projects is scope, ability, and making something fun. It’s entirely possible to make spherical planets you can walk around on in UE4, Astroneer is a great example. Looking at a lot of different games that have entire planets, you really get to see the limitations. LODs and view distance limitations are always obvious. It’s basically impossible to have a large number of assets that fit the planet well, they are going to look barren or repetitive or both (No Mans Sky). Star Citizen is probably the closet thing to your dream project, and they’ve basically had to rewrite most of CryEngine, and from what I’ve seen, there’s still a lot of bugs with the multiplayer aspects even in space, due to scale. And they have $137 million in funding, probably about 100 people working on the game, 5 years in development, and the game is still far from complete. Even though planet exploration isn’t their main focus, they still spend more time, experience, money, and resources than you’d be able to.
My suggestion would be to either
A. Try to make a working spherical planet for fun, and don’t worry about having a realistic scale or MMORPG elements.
B. Take the RPG elements from your dream game and work on making a good RPG without the planet aspect or the MMO aspect.
C. Make a simple multiplayer project where players can interact with the world and each other.
Limit your scope and goals to something obtainable, and don’t try to do everything at once.