Have a standalone small BD player and connect it to the pc and turn the pc off? Would this work? Blue Ray has not found its way into my home mainly due to all the artificially made problems. Do those boxes use hdmi or what?

ADD: I do not use my computer for other things if I am watching a film.

Assuming the monitor has HDMI, that should work. The only downside is if you wanted to also be doing work on the screen at the same time.

This is probably the easiest solution as Bluray players are a lot less expensive now. The main downside is that you can't back up the discs and you're supporting this anti-customer behavior.

Some monitors have picture-in-picture, or can split up the screen space between two or more inputs.

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Interesting, I haven't seen picture-in-picture in a very long time.

I've never seen a monitor that could accept inputs from different ports at the same time.

Gentoo and Arch wikis are sufficiently covering the question. Mpv since February 2018 supports protected bluray, so SMPlayer should be able to parse it.

DVD playback has the exact same problem. You need to circumvent the DRM.
Which is why distributions like Fedora, which care about such legal issues, don't ship the necessary library (libdvdcss).
see here: https://fedoraproject.org/wiki/Multimedia/DVD

Legal playback of DRM encumbered formats is simply not possible with free/open-source software.
Using open software to access DRM-protected media would always invalidate any protection the DRM is supposed to give.

So, how do I jack my linux so that I can watch my legally bought blu rays on it?

You buy windows and wait.

I'm pretty sure 2020 is going to be the year of Linux

You kill Bill Gates or something.