I've been researching and thinking more about what I'd really like, what you said about using a portable Oppo HA-2 headphone amp/dac and that marktomaras had also suggested using a portable amp/dac. Well, it's taken longer than it should have, but both of your very good advice has finally sunken in and I'm thinking this may be the best method.
I could set this all up on the rather large end table next to my current listening chair through one of my laptop's USB ports. I would just need to run a usb cord from the laptop to the amp/dac, install any necessary drivers and plug my headphones into it. ... and wallah, instant semi-wireless headphones that give me access to my entire collection of flac files on my Synology storage device. This is a little different than I originally envisioned it since my music files would be converted to analog via a different dac, which may not sound as good as the Oppo's excellent internal dac, and it still wouldn't allow listening to movies or tv through the headphones but I think this still may be the best solution thus far.
I could still run a quality 25' headphone extension cord to my other chair for the times I'd like to to have cable tv, blurays or even my music files converted to analog via the Oppo's internal dacs and listen through headphones.
After reading and watching numerous reviews on the portable and desktop amp/dac products currently available at reasonable prices, however, I think the better choice for myself would be a desktop unit preferably combined in a single case but also acceptable as separate amp and dac. I've discovered 2 desktop headphone amps from Aune (pronounced "ow-knee") I'm considering buying:
One is the Aune X1s a desktop amp and dac combo in a single case. www.youtube.com/watch?v=vlXW20ptu7E
The other is the Aune X7s pure class A headphone amp only, so I'd need to add a dac.
Both run about $300 on Amazon and I'm leaning toward the X1s because it's a combo but I'd buy the X7s if it included a dac.
I may just buy a pair of Sony headphones (MDR-1A or MDR-Z7) and use the small and inexpensive Audioquest Dragonfly usb dac. No headphone amp but unlikely it's needed with either of the efficient Sony headphones.