There is a LOT to learn to start self-hosting, that's for sure. With most stuff you definitely need at least basic knowledge of how your network functions, ports, NAT, and that kind of stuff.

Plex external access needs a port opened in your router to the plex server, it will try and do that automatically with UPnP but in some cases that's disabled and you need to setup NAT (port forward) manually. Your ISP could also be using CGNAT which means you can't open ports at all, but buying a static IP should make sure that's not the case (although you certainly don't specifically need a static IP for Plex). Often the message in Plex is wrong and will say remote access isn't working when it is in my experience.

Depending on your router/modem there may be some kind of security system running on it and that could be interfering with Plex as well.

Doing some external testing with a port scan on the plex port against your internet IP might help tell you something to figure out what's going on.

The *arrs not moving files or picking up watch folders is usually a permissions issue between the user they are running as, and your file/folder permissions for the source/destination folders. Windows definitely makes this simpler as you don't run into that issue as long as you're using an administrator account that has access to everything.

I've never used tdarr, but wouldn't your DVD rip software already be doing the encode for you anyways? You can also configure your DVD software to name the files and place them in the plex media folder, instead of the extra step of using radarr/sonarr to do it, since those are more for internet download organization.

Plex can also grab subtitles for you, at least that's how I manage mine.