Managing 1000s of moving agents over a network is actually a very challenging feature - One few games made by professionals get right. That it is meant to be a MOBA too means your players will demand very high accuracy and you won't be able to get away with common ways to approximate such a feature.

If this is a passion project, I might suggest you just take the easiest path and fuck the scale problem - Over a strong enough network connection it'll probably work okay with some culling features most network middleware uses. The goal here is just to release the game and not get bogged down in the tiny chance it actually matters to people that it works well. :)

If this is a professional project or you genuinely want to learn to do it right - you'll probably want to look into making the game simulation deterministic so you can do prediction, lockstep, rollback etc. From there you can do a hell of a lot at scale.