I tried Plex before, but I kinda liked the customization options / skins I could use with Kodi. The downside of using Kodi is that it's not a true media center, particularly if you run multiple clients and want a central library/database.
The compromise that's worked for me is with Emby, where I get a lot of the benefits of Plex (central database, my viewing history is stored on a per user/account basis, enabling me to watch using one client, stop, and pick up where I left off using another machine), but still get the skin customization flexibility of Kodi (using the Emby for Kodi addon)
Same scenario as OP, with my Emby server installed on my QNAP 653A, streaming to a mix of local and remote clients using a mixture of Minix U1 and Nvidia Shield boxes running Kodi. For those of thinking going the NAS route, it's definitely not cheap, but it's pretty convenient to have everything in one box that's permanently on, that runs all your downloads, and if your connection is fast enough, streams your media externally. I don't have a view on pros/cons of Synology vs QNAP as I've been used to QNAP (been using various models for 18 years now), but most applications would have versions for both.
and if you do have a NAS, tons of ways to automate your media. I use Sonarr to automate my TV shows downloading, and Couchpotato for movies. I don't rely on torrents which can be less reliable, but use usenet (but you have to pay a usenet server subscription), and NZBget for the actual downloading. sounds complicated, but it's not as difficult as you think, and once set up, it's great.
obviously a lot of debate about which media server is better, but at the end of the day, really depends on each individual's use case and preferences
some comparisons here -
https://www.smarthomebeginner.com/plex-vs-emby-comparison-with-kodi/https://www.makeuseof.com/tag/emby-vs-plex/