If you don't have Plus Disk, then you won't have Plus Disk content, yes. If you DO get plus though, you can still use that save file, so it's not a problem.
If you're level 204, then you might have already completely finished all non-plus content? That's more than high enough level to beat all the postgame bosses without much issue.
As for investment, I wouldn't worry too much, Plus Disk will end up burying you in money again. Everything's a work in progress~