I've been providing Plex for about as long as Plex has been around, but I don't really use it myself. Plex is mostly for people outside my home, and it is because it has an easy external authentication system for access that I keep it. That being said, Plex has gotten worse and worse and worse and worse over time with more useless nonsense and features I actually liked being removed. I do have a Lifetime Plex Pass so I am at least not impacted by the recent demand that external users get a Streaming Pass for monthly access, but I still don't see it as a favorable direction for the project.
Jellyfin and Emby are home media servers with similar goals but neither has the external access component that makes Plex worthwhile.
Some hints:
Plex is much more reliable on a Linux host. Also, it really, really does run well on $150 Celeron N150 systems because the builtin graphics cores can handle a half dozen transcodes simultaneously. You want a recent-ish Intel PC to run your server. Don't bother running it on an ARM NAS; unless it's got a Tegra K1 or a premium Qualcomm SoC, it won't have the power to transcode at all.
Make Granular Libraries. This is a general library management tip, but I've found it useful to have the classic TV Shows and Movies, but also Kid TV and Kid Movies (handy for manually enforcing age restrictions), TV Documentaries, Movie Documentaries, Stage Performance (I have a lot of recorded operas and ballets), Exercise, Tutorial, Stand-Up Comedy, TV Anime, Movie Anime, K Drama, Dropout.TV, Youtube Downloads and Home Movies. Because of how I keep things, I also have Disney, Star Wars and Super-Hero content broken out as libraries simply because those things get used very frequently.
Use Kometa to provide more information about your content. Kometa adds little badgets to the posters in Plex and autogenerates playlists, so for example it creates lists of all the TV shows you have from each TV network or streaming service and indicates whether movies have won awards.
Pay attention to file naming conventions. {Edition-4k} is handy or {Edition-Blade Runner Super Special 2nd AD cut 40th Anniversary}. Filebot is very helpful for keeping file names in check if you don't know where to start with that.
Some TV shows are extremely difficult to get properly named for Plex. Battlestar Galactica and Adventure Time are poster children for this. Check how things are named on the TVDB for those guys and conform to that. In some cases (VERY SPECIFICALLY Doctor Who), the debate over order is so divisive and I've told my users that it will just never be available. Plex will never get it right to the satisfaction of its fans and I got tired of my users bitching about it.
Don't bother using Plex for Music. Plex says it is good at music. Plex is LYING.
As I said, I don't really use Plex. I like being able to control presentation better via Kodi, which also DOES handle music better, but it doesn't really work outside my house. I do pay attention to it. I want it to work for other people.
Jellyfin is the guy all the open source folks like. It's fine but it doesn't have the wide support on smart TVs and it doesn't have easy remote access. You could roll your own but personally my uncles have a hard enough time with the concept of opening a browser and visiting a web page to type in a code, which is all they need for Plex to work. I don't want it to be any harder than that.