Yes, I do plan on using a Samsung T5 SSD drive instead of the Fusion drive.
The T5 will be fine. Not as fast as the internal SSD, but overall, most likely a better experience than the Fusion Drive with the tiny 32GB SSD portion.
A downside of the T5 is that you will not have TRIM support over USB, which could lead to problems over the very long term for an external USB SSD drive, but there are plenty of people that have been using USB drives to boot from for years without any major issues.
Also, while using an external drive might reduce the chance of HDD failure, and can still happen. Sometimes when this happens, the HDD failure could cause booting issues even when booting from an external drive. This personally wouldn't stop me from getting a Mac with a Fusion Drive, but I still think people should know that HDD failure could still happen even if you do not use the drive.
I don't want to be a party pooper here, and I know this is off topic, but using an old 2006 Mac Pro as an always-on Plex server will really chew through your electricity bill. As in, running your 2006 MP will cost > 1 Mac mini per year. I'd rather buy a new Mac mini every year. Once I did the math on my end I got rid of my old pro, and my wallet is thanking me every day.
How much to you pay for electric?
Not sure where you live, but energy cost are very cheap (relatively) where I live. You are right though, the Mac Pro does a lot more energy than the Mac Mini, but not even remotely close to the cost of the cheapest M1 Mac Mini over a year. Maybe a half-decade's worth of energy.
I know approximately how much it costs me to run my Mac Pro 1,1, as I tested power consumption on it and most of the electronics I use in my home.
At idle, it costs a little over $0.40 a day for it to run 24/7/365, or a little over $150 a year. So, just enough to pay the tax on a new Mac Mini purchase and have a little left over a few meals.
Another thing to keep in mind is that I would have to have external drives for the HDDs plugged in to the Mini, all using energy, so it isn't not as simple as comparing the energy usage from the two machines, and the Mac Pro contains the drives for the server.
That said, I have been thinking about moving my Plex server to my Mac Mini for a few reasons, one of them being energy, but others as well.
Prior to using my Mac Pro, I was using my Late 2012 iMac as my Plex server, but was getting strange glitches. I had a Mac Pro sitting around collecting dust, so I decided to use that. It has many drive bays which made upgrading my storage HW super easy, and I run it headless so no added energy usage there.