Your Oppo should play 50GB DL bluray discs, and if so, I can point you in the right direction.
Register for the forums over at thestarwarstrilogy.com
Register in the FORUMS section, not just the main page.
At least for me, my forums login doesn’t access the main page.
In their download section there is information on how to download many different versions of our beloved trilogy.
There are 1080p ~40GB bluray ISOs in there for 4K77 and 4K83.
That’s where I got mine.
I even accessed usenet, but all I could get there were mkv files and like you I wanted the ISOs.
For the menus, language options, subtitles…
We’re not allowed to post links outside of their forum, so I can’t give the exact information required on here.
Check them out and see if this helps.
Good luck.