I've been running a full tower Windows server with a dozen drives for a decade and decided to downsize. This ministack does everything I need at a fraction of the power, noise, and heat.
I use it primarily for Plex but also host a few games servers for buddies. It fits perfectly in my entertainment center.
Spent around $200 on the Mac Mini plus $600 on the drives and enclosures. I'm using SoftRAID for RAID1.
The problem isn't 4k vs 1080p. I have a separate library for my 4k items which isn't accessible remotely.
The problem is twofold:
My uplink speeds suck, so often a 1080p file can't stream remotely without being transcoded down to 720p.
Almost everything I have is encoded to x265, and many Roku devices don't support that, thus causing media to be transcoded even locally. (The device I use for 4k stuff does but other TVs in the house currently use devices such don't)
Also, I'm not just running this on a raspberry pi with a single disc. I have over 1500 movies, and 200 TV series already taking up over 30 terabytes on a RAIDZ2 server. So it's not just a simple matter of throwing an " extra fifty bucks" at it.
Frankly, with everything that's running in my home lab, the added electricity in CPU power to transcode is barely a rounding error.
Is it possible for me to arrange things so that nothing ever needs to transcode? Sure, but it would be far more trouble and cost than it's worth at this time.
Not everyone has the same needs or restrictions.
How bad is your upload speed? Most of my x264 and x265 encodes have a total bitrate of around 1.5-2 Mbps. You can't stream that without transcoding?
Also, have you LOOKED at transcode quality from the other side? It looks TERRIBLE, and it doesn't significantly reduce your bitrate. I'm telling you, whatever math Plex does to decide whether or not to transcode is USELESS. I've seen it transcode x264 videos (that definitely didn't need to be transcoded) and send them to friends at a HIGHER bitrate, it's legit insane.
Do what works for you, but Plex transcoding is a bad "feature" that should be avoided at all costs.
EDIT: I'm far from the only one noticing this, the common wisdom is that Plex clients are telling your server to transcode, even when it's not necessary, and there's not much you can do about it other than fully turning transcoding off.
https://www.reddit.com/r/PleX/comments/ojsspo/help_finding_out_why_plex_is_choosing_to/
https://www.reddit.com/r/PleX/comments/ugu0rg/how_to_avoid_transcoding_as_much_as_possible/
https://www.reddit.com/r/PleX/comments/13qrknp/plex_transcoding_when_it_shouldnt_advice_please/
https://www.reddit.com/r/PleX/comments/uxu4v6/transcoding_when_it_shouldnt_be/
https://www.reddit.com/r/PleX/comments/crhw8r/why_is_plex_transcoding_when_it_doesnt_need_to/
https://www.reddit.com/r/PleX/comments/9qade9/how_do_i_avoid_transcoding_as_much_as_possible/