LR-ACR & uncovered ground

In LR the ACR (YES LR has Adobe Camera RAW. It is Built Into LR) edits are store in the Catalog file by default.

If you want to see the LR edits on the image when opened in the Adobe Camera RAW plugin then you need to either turn on Auto Write changes to XMP in the catalog Settings dialog or do a Save Metadata to file found in the Metadata menu item or use the keyboard shortcut Ctrl + s.

Then when opened in ACR you will see all of the edits you did in LR.

In ACR you can zoom to 1600% and in Lightroom to 11:1 and use the adjustment brush.  How much more accuracy do you need?

In transform you can change the grid size to a very small size

To change the spacing of the grid that is overlaid in Transform, hold the Ctrl key (Windows) / Cmd key, and a little control panel appears at the top of the image area. Within this you can change the grid spacing to a larger or smaller number (I think the number is screen pixels) and also change the opacity of the grid overlay.

As for precise brushing, LR is largely designed with a fairly broad-brush (pun only partly intended) working method in mind. The nature of the adjustments applied by a brushed mask as well as the tools offered for making and altering masks, are tuned to be as un-finicky as possible and that carries with it, certain limitations for those trying to work in the same way they might in (say) Photoshop. Many people will turn to Photoshop itself for that kind of requirement. Such as extractions etc, which are outside of LR's remit.

I summarise this as: Lightroom for adjusting, Photoshop for altering. Under that conception, only 'altering' really ever depends on achieving a precise edge IMO. An adjustment can however be set to affect only certain tones in its action, either natively (e.g. by choosing say Highlights rather than Exposure) or else via the range masking feature. And that helps obviate the need for a really detailed mask to be made in the first place - it can be left much more approximate.

That said, a pretty respectable mask can still be achieved in LR via Automask and the "dab" method - i.e. largish highly feathered brush, clicking in place rather than "drag painting", whereby a similar selection gets added / subtracted for each new click,  based on whatever colour's right under the brush crosshair each time. A heavily adjusted version of the image may help in achieving such a mask within this technique, either with the desired image adjustments reinstated from history once that's done, or else via a temporary virtual copy which is then thrown away.

I cannot comment on the size of the brushes in Lightroom or Camera Raw. That is something that you will have to work out on your computer. However, there is no correlation between Lightroom and the Camera Raw plug-in because they are completely independent and do not work together. Camera Raw is for Photoshop and is not connected with Lightroom other than to facilitate the transfer of raw images from Lightroom to Photoshop.