Isometric Grid

https://rj-graffix.com/downloads/plugins/

This free plugin helps

but the snapping system on illustrator isn't too smart

eg making a rectangle sometimes you have overshoot one of the lines and drag it back

or delete the extra with the ShapeBuilder + Alt  or Option

Isometric mover tool is good too, lets you drag the  whole face at correct angle

shows a nice guide line too

Though you have to Edit >Copy and  Edit> Paste in front

, first  if you want to move and copy, as the Alt key is for free movement

Illustrator has no provision for axonometric grids.

But frankly, in the 40 years or so that I've been doing isometric drawing, both "on the board" and in software, I've never been dependent upon grids, and have never known another serious axonometric illustrator who is, either.

As usually implemented, page-spanning grids are a crutch for trivial isometric drawing. Beyond the trivial, they are just cumbersome.

Functionally, they try to substitute for the freely-moveable scales affixed to "head" of the track drafter mounted on an engineering drawing table in pre-computer days. It's a poor substitute. You've already alluded to one of their problems: The increments are only useful when they correspond to measures of the subject you are drawing. Another is that in axonometric drawing, you are never concerned with measuring from the page origin; you're concerned with measuring from a pre-existing point in the drawing. So such grids should always have the ability to fluidly follow the cursor, and snap the nearest major increment to snap candidates on the page.

Instead, acquaint yourself with the Smart Guides feature, including its options, features, and keyboard modifiers. Set its Construction Guides to 30° angles.

JET