The quickest solution is to get everyone out of the file, open detached with audit, purge till nothing is left to purge, save. Try that 2-3 times. Then detach one last time. Archive or delete ALL file data for the central file in question. Backup file, and project file. Save a NEW central file and let everyone in, reminding them to create new local files when they open.

Well made model elements being replaced by detail lines is a scary solution. You're basically ruining the integrity of the model. Your cad links or imported cad will cause more performance trouble than actual well made families ever will. Now in-place families could be a problem as they are stand alone modeled geometry that is crazy inefficient.

What isn't clear is if you are on a revit server or just a plain windows server. If revit server you might need to go into the server and clean out and reset some of the database. Much more complicated than that and I doubt you have access (not to the website, have to log into actual server).

Regarding groups/links. Both will cause issues at some level. Links get *very* sluggish after a while with so many instances. I think Twiceroadsfool has some posts on best practices with groups. But in general that type of project, unit based buildings (apartments, hospitality, hospital rooms), are just plain hard to deal with.

In general, I'd start with auditing, purging and repeating 3 times. Remove old backup data and project file, create a new central. IN the process I'd try to remove any resolved design options (pick primary, accept, which deletes alternate), remove in-place families, hunt down and destroy any cad I could, purge some more, verify worksets are being used properly (links in worksets, both by type and instance, reduce to necessities). Check groups for nesting. I think best practice is 3 deep at best but others have the details on that.

Good luck, you're walking into a mine field if you are just now on the job. A 2 year old file should get some serious maintenance before progressing, especially at this size.