Now let's say I want to watch a movie on my TV.
I can rent a 4K blu-ray locally for 3€ per night and that'll work just fine. The Blu-Ray player needs regular online updates and is in general pretty hostile towards me, the law-abiding consumer, but it works. I've also heard that people have a great experience with RARBG, which appears to be a (likely illegal) place to freely download 4K movie rips. Then there's iTunes, where I can "purchase" and "download" movies, but the resulting files won't play on my TV, because they are wrapped in iTunes DRM. Also, iTunes vehemently refuses to download 4K movies, so for those I would be at the mercy of a good internet connection. Now I've heard that it is technically easy to rip the iTunes DRM to recover an MP4, but that is likely illegal again.
It appears that rented blu-rays are the only reasonable option here, but for convenience, I would like to download new movies. But legally. I don't mind if the file has DRM, as long as I can make the TV play it fully offline. So that means Widevine should be fine, as it's an Android TV. But it appears that all Widevine-based movie services (iTunes, Hulu, Amazon, Netflix) refuse to let me download 4K files. Also, they all insist on their own shitty apps, which I'm not sure I can get onto the TV without internet.
Is there any place where I can legally purchase a movie file that can be played offline on an Android "smart" TV?
I've read conflicting stuff about this, but my understanding is that it is technically legal to make "backup" copies of media you own. However, breaking encryption to bypass DRM on the blu-ray (which you will need to do in almost all cases) in order to make that backup is considered a violation of copyright.
So, while I wouldn't advise anyone to break the law, I will say that this is one of those laws that I don't think anyone has an ethical obligation to follow. It's not only a stupid law, but almost impossible to enforce. Make of that what you will.
I personally use the nvidia shield tv pro.
Play a Blu-ray on an Xbox or laptop connected to the TV instead of your bad player, it should be more convenient than downloading on your questionable connection and shuffling files around all the time. After all, it's just a digital file with DRM on a storage device, which you're ok with. There may be rights issues that limit stores to offering 4K streaming and not downloads.
If you do bother to switch to another playing device, maybe try grabbing a UHD Movies Anywhere movie then playing it in 4K from several of the Movies Anywhere linked services to see if one buffers better or has a better bitrate suitable for your connection.
If the underlying connection problem is the ISP in your area and it doesn't even offer 20Mbps, maybe you could give Starlink a try. If it's more about wireless signal or interference, look into a better wireless router or try ethernet.
Lastly, the HD releases of some movies now are quite good due to better source material, especially at some viewing distance/screen size combos. I know it can feel like a waste when you've invested in a great TV since you miss out on HDR or other features, but part of having a great device is good upscaling. How much it matters really depends on the kind of movie.
You’d avoid the forced upgrades on your LG and suffer a minor inconvenience of streaming from another device which does have internet access. I don’t know what the ad’s situation is on those devices, but I’m sure with some research you could find a streaming device that hopefully meets your requirements.
Another option is cast all the videos from a tablet or computer to the TV.
Alternatively, as far I know, your options are buy ‘n rip or pirate them.
IMO the “smart” features of basically all TVs are worse than useless and trying to copy media to the TV is a huge waste of time compared to using streaming hardware (Apple TV, Roku, etc)
International waters.
I, like you, also have an LG OLED, and I use RARBG. I outlined my setup in a previous comment:
I'm using rarbg as my main source which has magnet links that qBittorrent detects on click. I use mpv with Anime4k for animated content or an Nvidia Image Scaling glsl filter which is a decent upscaler/sharpener. I also use SVP for frame interpolation with an RTX 3080 which supports optical flow, and I assume it'll be even better in the RTX 40 series given their DLSS 3's frame interpolation exclusivity. It all looks incredible on my LG OLED TV/monitorm with 4k HDR 10 bit support.
As for the legal part of your question, I firmly believe uploading content is different than downloading it, so I have no issues sailing the high seas. Especially considering all of the price fixing scandals and Hollywood accounting that end up as a hidden tax on consumers. I have no moral qualms looking at something for free.
But then again, maybe there's some entrepreneurs in here who can make it happen ;)
Looking at RARBG, it should be possible to convince movie execs that no, DRM is not preventing piracy. And I have a feeling that the "I go through extra work to avoid ads" crowd would have a higher than average income, so for them, $30 (the price of a recently released bluray) for a HQ file might work.