For filesystem, I’d recommend ext4 for all. FYI since you are coming from a Windows environment, you won’t need to defrag your HDD disks (or at least only very rarely) on ext4 since it is more intelligent about placing files. If you get a SSD I’d recommend mounting it with the “noatime” option. By default Linux records the last read time, so you actually get a small write with every read. Mounting with the “noatime” option turns this off and saves some wear on your SSD.
I’ve got my OS on an SSD , meaning that root, or “/”, is on the SSD, but I moved /home to a HDD. Outside of your media, /home is where you are going to see the most size growth (and outside of OS upgrades, also the most writes) because that is where pretty much everything specific to your user is written. Sure it’s good to have /home on the SSD, but throwing it out there since you said you might get a small SSD… Absolute best performance of course is to have everything on a bigger SSD, but my mix of OS on SSD, and /home on HDD is still fast. Lots of info out there and opinions on both sides if you want to read up to make your decision.
If you are going to use all three drives you listed for media, think about what your Plex libraries are, their size, and how you expect them to grow to help plan how you want to store your Plex libraries on those disks. Each disk (unless you decide to have multiple partitions on each disk) will be mounted at one specific mount point in your Linux file system “tree”. Plex does have the ability for a library to be mapped to multiple directories (and thus potentially multiple disks), but for simplicity I would suggest if possible targeting at the start to have each library have only one directory, and thus one disk. Certainly you can have subdirectories for your disk mount points and therefore multiple Plex libraries on a disk. (I’m a little tired, I hope that makes sense.)
Learn and use Timeshift. It will save you headaches when you do something that doesn’t work and want to roll back.
In general allow yourself time to read, research, and learn when you run into an issue. There is lots of info out there and always something new to learn with Linux if you enjoy the process.
Welcome to Linux!
Which I7 specifically? It’s great you are happy with performance. Be aware that different generations have different GPUs, and some specific models don’t have a GPU. If you have a good GPU you can offload Plex transcoding to it, which will make your overall performance better.
Edit: For backups to external drives look at rsync. Similar in some ways to Windows robocopy if you want to write a backup to external disk that just copies over new/changed files, and if you want it can delete files from the external drive if they no longer exist in the source.
