You can create a bookmark via Insert → Bookmark
and link to it later in the document with Insert → Link and select the bookmark:
You should probably give it a more meaningful name than I did, e.g. "Definition: <word>".
I also had this happen to me once, using Backup and Sync. I also stopped the process, like you did, to prevent the modified date from being changed. It's a bug—I don't know how one solves it. Yes, the modified date is changing on the cloud, and will change on other machines as well.
However, the fewer files you sync, the less chance that something will go ...
Unfortunately, there are no options provided for checking folder sizes whether they are shared or not. There is a whole thread at Google Support since 2019 asking how to check (normal) folder size. However, Google has not yet implemented the feature.
As this feature is available to all accounts, you are missing something but we can't tell you exactly what: some Google Editors1 features:
are only available in Chrome.
requires to be signed in a Google Account
requires a G Suite account
Please bear in mind that you could send your feedback directly to Google through Google Feedback 2
While posting I noticed two tiny words "and enforced".
You must have 2-step verification enabled and enforced on your account
It was enabled, but not enforced.
Once I enforced 2FA for the org, I was able to see the "START EXPORT" button.