NVMe w/ enclosure vs Samsung T5 vs Samsung T7 - External SSD for 2018 MacBook Pro?

Let's get some things straight. You wrote, "The 2018 MacBook Pro has FW3 ports..." What is FW3? USB 3.x is not the same as TB 3. USB 3.x ports support up to 5Gb/s transfer rate. TB 3, on the other hand, could mean 5Gb/s, 10Gb/s, 20Gb/s or 40Gb/s transfer rates dependent on the disk controller and/or the bridge interface of the external enclosure. You need to carefully read all the information provided for an enclosure. It may say TB 3 but that may mean the port is USB-C. Your MBP has full speed TB 3 ports that support transfer speeds up to 40Gb/s. However, to attain that speed you will need a special TB 3 enclosure with specs that state it supports 40Gb/s. Note that if you install the NVMe SSD in an enclosure that supports 40GB/s TB 3, then it will not be usable on any other computer that does not have a 40Gb/s TB 3 port.

I've used several Samsung 970s as external SSDs on an iMac and a MBP like yours. They work quite well but the TB enclosures are expensive. I've purchased mine from Amazon. I've used one by Wanfocyu and another by Blueendless. They cost somewhere around $130.00. Play to spend that much or more for a good enclosure capable of 40Gb/s TB 3 speed. These two are the same as this one: Thunderbolt 3 SSD Enclosure, Shockproof Aluminum External Case TB3 Adapter for M.2 NVMe Solid State Drive PCIe 3.0 x4 40Gbps with 100w Cable (SSD Not Included). There are a couple of others listed on Amazon that do support 40Gb/s and run up to $170.00, or so, in price. There are others posted at similar prices but do not state they support 40Gb/s. Don't buy them, if this is the speed level you want.

Well, USB-C ports also support TB 3 at the speeds supported by the hardware (including the enclosure.) If you were happy with USB 3.0 speeds of 5Gb/s, then almost all cheap enclosures will work. If you want USB superspeed (10Gb/s) then be sure the enclosure specs state that. You can spend $30.00 on an enclosure that supports 10Gb/s, as well as another that doesn't.

Also, why spend the money on a 970 when an 860 or 870 might work just fine in an inexpensive enclosure at 10Gb/s from a USB-C port. They won't provide the 2500MB/s r/w speeds of the 970 but do quite nicely at around 900-1000MB/s in a decent $30-50 enclosure.