Is there a limit to how large a position value can go?

A float is a long number with a decimal point, so if it holds a very large number, it loses decimal places.

What this means in a practical sense is objects with a very large transform position have a correspondingly lower accuracy, and will start clipping into each other and z-fighting.

I think Unity performs a translation from world to camera space before rendering to help avoid this, but that won't magically create the missing decimals.

I would try changing the import scale down on the models; it may be time-consuming but the other drawback of very large units is a lot of systems assume 1 unit = 1 metre, and you'll likely run into issues with physics & animation down the road which whilst fixable mean you end up writing scripts designed to work with crazy scales, and which thus aren't transferable/reusable. A lazy fix is to put your environment as a child of a single gameobject and set it's scale to the inverse (e.g. 0.001).