The circle part could be decal, but using decal trim textures instead of one giant one, similar to Star Citizen decal technique you can read about here: https://polycount.com/discussion/155894/decal-technique-from-star-citizen You can also consider since this is such a massive object modeling a bit more geo into it to…
Unique macro texture for distant lod covered by small repeating geometry pieces , tileable materials in segmented mesh and decals covering seams at close , and hiding or replacing low res macro to certain extent . Probably only at the bottom part where players could come nearby.
Here, an example. We're assuming that the player WILL be able to get close to every part of the structure. So you can't just disregard the texture quality above 5 meters, because "the player will never see that anyway". The player will see that, because they can climb it. I guess the main block of the structure could be…
There are always multiple ways to achieve a certain result. But here's how I'd do it and how we do it in production (also they did something similar for God of War, there are many posts on Artstation): Create the structure as one big mesh - since it looks very stone-ish, bring the whole thing into ZBrush for a sculpting…